Drug Half-life Explained

Medically reviewed by Carmen Pope, BPharm . Last updated on May 23, 2022.

Drug Half Life / Clearance Calculator

Work out how long it takes for a drug to leave your body.

Half life Clearance

This is an estimate on the time it will take for a drug to be removed from the body.

The actual half-life of the same drug may vary significantly from person to person, because it depends on a number of different patient-specific and drug-specific variables.

This tool should NOT be considered as a substitute for any professional medical service, NOR as a substitute for clinical judgement. It should not be relied upon to predict the time period required to ensure a negative drug test result, because laboratory tests mostly test for drug metabolites.

What is the half-life of a Drug?

The half-life of a drug is an estimate of the time it takes for the concentration or amount in the body of that drug to be reduced by exactly one-half (50%). The symbol for half-life is t½.

For example, if 100mg of a drug with a half-life of 60 minutes is taken, the following is estimated:

  • 60 minutes after administration, 50mg remains
  • 120 minutes after administration, 25mg remains
  • 180 minutes after administration, 12.5mg remains
  • 240 minutes after administration, 6.25mg remains
  • 300 minutes after administration, 3.125mg remains.

In theory, we can see that after 300 minutes, almost 97% of this drug is expected to have been eliminated. Most drugs are considered to have a negligible effect after four-to-five half-lives. However, this does not mean that won’t be detectable, for example, during a drug test. Just that they will have no effect.

Drug Half Life Graph

In reality, the actual half-life of a drug varies from person to person, because it depends on a number of different patient- and drug-specific factors. These affect how well a particular drug is distributed around a person’s body (called the volume of distribution), or how fast a person excretes that drug (called the drug clearance). For example, the IV drug gentamicin, which is cleared through the kidneys, has a half-life of 2-3 hours in a young person with no kidney disease, but its half-life is over 24 hours in somebody with severe kidney disease.

Generally, it is difficult to precisely say how long a drug or substance will take to be excreted from someone’s body. This is an important fact for athletes or people in occupations that require them to be substance-free to remember. Half-lives in the anti-doping world are of limited value because they do not reflect the presence of metabolites (break-down products from the parent drug), which are often what is measured in anti-doping tests. In addition, serum half-life does not necessarily reflect urine concentrations, which is the main way they take samples for drug testing.

Patient-specific variables that may affect half-life

  • Blood circulation
  • Diet (eg, grapefruit juice and several drugs, green vegetables, and warfarin)
  • Excessive fluid (such as in people with heart failure or edema) or low fluid levels (dehydration)
  • History of previous drug use
  • Kidney function (for drugs that are cleared via the kidneys)
  • Liver function (for drugs that are metabolized through the liver)
  • Pre-existing conditions (such as heart failure, gastrointestinal disorders, pregnancy)
  • Presence of drugs that compete for binding sites or interact in other ways
  • Race/ethnicity or genetics (this can influence the metabolism of a drug)
  • Other variables, such as if the person is on hemodialysis.

Drug-specific variables that may affect half-life

  • Drug formulation (ie, modified or controlled release preparations extend half-life)
  • How the drug behaves in the body (ie, zero-order, first-order, or multi-compartmental pharmacokinetics)
  • How the drug is administered (half-life may be different with IV administration, compared to intranasal or oral administration)
  • How the drug is cleared from the body (eg, kidneys, liver, lungs)
  • If the drug accumulates in fat or other types of tissue
  • If the drug binds to proteins or not
  • Presence of metabolites or other drugs that may interact
  • Properties of the drug, including molecule size, charge, and pKa
  • The volume of distribution of a drug
  • Other variables, such as if the drug is actively transported, is self-induced, or has saturation pharmacokinetics.

Short versus long half-lives

Drugs or substances that have a shorter half-life tend to act very quickly, but their effects wear off rapidly, meaning that they usually need to be taken several times a day to have the same effect. Drugs with a longer half-life may take longer to start working, but their effects persist for longer, and they may only need to be dosed once a day, once a week, once a month, or even less frequently.

When considering drugs with a high addiction or dependence potential, those with a short half-life are typically harder to withdraw from than those with a long half-life. For this reason, drug treatment programs will often switch a person from a short-acting drug to a long-acting equivalent from the same class, in order to improve the withdrawal process.

List of common medicines or substances and their half-lives

* Note that half-life varies depending on the source used. Half-life in this table refers to the elimination half-life.

  • Are expired drugs still safe to take?
  • Can grapefruit juice interact with my medications?
  • Common Drug Side Effects
  • Generic Drug FAQs
  • How do I remember to take my medications?
  • How do I stop my medication safely?
  • How to Safely Dispose of Your Old Medications
  • Imprint Code FAQs - For Oral Medications
  • Injection Types and Sites
  • Medical Conversions - How many mL in a teaspoon?
  • Pill splitting - Is it safe?
  • Prescription Abbreviations: What Do They Mean?
  • Top 5 Ways to Avoid Drug Errors
  • Top 9 Ways to Prevent a Deadly Drug Interaction
  • What are pharmaceutical salt names?
  • What are the risks vs. benefits of medications?
  • What is the placebo effect?
  • What is a drug's half-life? Mind UK. https://www.mind.org.uk/information-support/drugs-and-treatments/medication/explaining-the-half-life/#:~:text=What%20is%20a%20drug's%20half,few%20days%2C%20or%20sometimes%20weeks .
  • Hallare J, Gerriets V. Half Life. [Updated 2021 Aug 23]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK554498/
  • Pharmacology: Half-life of Drugs School of Health Sciences. The University of Nottingham. https://www.nottingham.ac.uk/nmp/sonet/rlos/bioproc/halflife/index.html

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

Medical Disclaimer

' width=

Select a Community

  • MB 1 Preclinical Medical Students
  • MB 2/3 Clinical Medical Students
  • ORTHO Orthopaedic Surgery

Are you sure you want to trigger topic in your Anconeus AI algorithm?

You are done for today with this topic.

Would you like to start learning session with this topic items scheduled for future?

Pharmacokinetics

https://upload.medbullets.com/topic/107002/images/volduistrib.jpg

  • V d = (amount of drug in the body) / (plasma drug concentration)
  • ↑ V d of drugs
  • via ↓ protein synthesis
  • via urinary protein loss
  • ascites, pulmonary edema, heart failure
  • can lower plasma concentration of water soluble drugs
  • drugs distribute in vascular compartment (blood) and bind plasma proteins
  • drugs are large and/or charged molecules
  • drugs distribute in extracellular compartment and/or total body water
  • drugs are small, hydrophilic molecules that do not bind plasma proteins
  • > body weight
  • drugs distribute in all tissues
  • drugs are small, lipophilic molecules that bind strongly to extravascular proteins
  • K e = elimination constant
  • CL relates the rate of elimination to the plasma concentration
  • t 1/2 = (0.7 * V d )/CL
  • a drug infused at a constant rate reaches about 94% of steady state after 4 half lives
  • for drugs eliminated by first-order kinetics, half-life is constant regardless of concentration

how to do half life problems pharmacology

  • bioavailability is defined as unity , or 100%, in case of IV administration
  • e.g., orally, F = percent that is absorbed and survives first-pass metabolism in liver
  • Calculation of bioavailabillity (F) = 100% * (AUC-oral * Dose-IV) / (AUC-IV * Dose-oral), where AUC is the area under the curve of a pharmacokinetic plasma concentration versus time plot
  • delayed release formulations will have slower rise and lower peak compared with rapid release formulations
  • Pharmacology
  • - Pharmacokinetics

Please Login to add comment

 alt=

U.S. flag

An official website of the United States government

The .gov means it’s official. Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

The site is secure. The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

  • Publications
  • Account settings
  • Advanced Search
  • Journal List
  • HHS Author Manuscripts

Logo of nihpa

The Operational Multiple Dosing Half-life: A Key to Defining Drug Accumulation in Patients and to Designing Extended Release Dosage Forms

Selma sahin.

1 Department of Biopharmaceutical Sciences, University of California, 533 Parnassus Avenue, Room U-68, San Francisco, California 94143-0912, USA

2 Faculty of Pharmacy, Hacettepe University, 06100, Ankara, Turkey

Leslie Z. Benet

Half-life ( t 1/2 ) is the oldest but least well understood pharmacokinetic parameter, because most definitions are related to hypothetical 1-compartment body models that don’t describe most drugs in humans. Alternatively, terminal half-life ( t 1/2,z ) is utilized as the single defining t 1/2 for most drugs. However, accumulation at steady state may be markedly over predicted utilizing t 1/2, z . An apparent multiple dosing half-life ( t 1/2, app ) was determined from peak and trough steady-state ratios and found to be significantly less than reported terminal t 1/2 s for eight orally dosed drugs with t 1/2,z values longer than one day. We define a new parameter, “operational multiple dosing half-life” ( t 1/2, op ), as equal to the dosing interval at steady-state where the maximum concentration at steady-state is twice the maximum concentration found for the first dose. We demonstrate for diazepam that the well-accepted concept that t 1/2,z representing the great majority of the AUC will govern accumulation can be incorrect. Using oral diazepam, we demonstrate that t 1/2, op is remarkably sensitive to the absorption t 1/2 , even when this absorption t 1/2 is much less than t 1/2,z , and describe the relevance of this in designing extended release dosage forms. The t 1/2, op is compared with previously proposed half-lives for predicting accumulation.

INTRODUCTION

Half-life is the oldest pharmacokinetic parameter and all clinicians believe they understand its relevance. Here we point out that although the relevance may be understood, the actual value reported for many drugs is not the relevant half-life and that even today the appropriate method for determining the relevant half-life has not been adequately discussed in the literature. In a number of cases, well accepted and generally used approaches are just wrong.

Half-life is the time interval in which half of the drug in a system is lost (when drugs do not exhibit saturable or any other nonlinear, anomalous or fractal kinetics, which is the condition considered in this work). If a drug is dosed at a frequency equal to the drug’s half-life, the patient exposure to the drug at steady state during a dosing interval will be twice the exposure for a single first dose. If the dosing interval is less than the half-life then the exposure will be greater than double and if the dosing interval is greater than the half-life exposure will be less than double. But because drugs readily distribute out of and into the measured systemic circulation (blood/plasma/serum) from tissue compartments following a single dose all drugs will exhibit more than one half-life parameter. Thus, a problem arises when clinicians pick the half-life upon which to make exposure accumulation predictions, since drugs exhibit multiple half-lives. Usually the terminal (longest) half-life is chosen, but frequently this is incorrect. This work addresses this issue, which surprisingly has not been adequately explicated, considering the clinical significance of accumulation reaching either toxic or ineffective concentrations and its implication for designing extended release dosage forms.

It is well recognized that clearance divided by bioavailability defines the appropriate multiple dosing rate for a particular drug, i.e ., the mass of drug dosed per unit time. For a drug following linear kinetics, this multiple dosing rate will yield the same average systemic concentration, that is, the same systemic exposure, no matter what the dosing interval. For example, at steady state a 240 mg daily dose will yield the same average systemic concentration/exposure as 120 mg every 12 h, 80 mg every 8 h, 60 mg every 6 h or 40 mg every 4 h, and this average systemic concentration/exposure will be equivalent to the steady state concentration/exposure obtained with a zero-order infusion of 10 mg/h. The decision as to the appropriate dosing interval for that drug will depend on the half-life of the drug. Unfortunately, for most drugs there is no single half-life that can be readily accepted. This is due to fact that the systemic concentration time profile for most drugs is best described by a multiexponential function, thereby yielding more than one half-life to describe the drug. The choice of a single appropriate half-life value for such drugs is unclear. Often, the half-life describing the terminal log-linear phase of the log concentration time profile is selected as the single value to be reported. However, this terminal half-life may only represent a small fraction of the total clearance of a drug and, thus, is relatively unimportant in defining the accumulation of systemic concentrations upon multiple dosing. This uncertainty may be even further exacerbated when the drug is given orally and an absorption rate constant must be considered.

Probably the best example relating to the discrepancy between the terminal half-life and the operational half-life is for the commonly used anxiolytic drug diazepam, although to our knowledge this has not been examined carefully in the literature. If one looks up the diazepam half-life in Goodman and Gilman (43±13 h) ( 1 ), the Drug Information Handbook (20–50 h) ( 2 ) or any other standard reference book, a long half-life is reported. Yet the recommended dosing interval for diazepam is three to four times a day. Thus, one might suspect that the generally reported long half-lives are not relevant in achieving therapeutic efficacy for diazepam (we note that the package insert for diazepam tablets makes no mention of half-life).

Since, half-life is most important in defining multiple dosing regimens, it may be useful to describe parameters that expressly address this question. For a simple one compartment body model, there is only one half-life. Thus, if one were to dose the drug intravenously at a dosing interval equal to the half-life, systemic concentrations at steady-state would drop 50% during each dosing interval and accumulation would yield a maximum systemic concentration at steady-state that is twice the maximum systemic concentration found after the first dose. Thus for any drug following multi-compartment linear drug disposition, we define the operational multiple dosing half-life as equal to the dosing interval at steady-state where the maximum concentration at steady-state is twice the maximum concentration found for the first dose and where the fall off to the trough plasma/blood concentration from the maximum plasma/blood concentration at steady-state is consistent with this half-life.

Various approximation equations have been proposed for calculating the dosing interval. Benet ( 3 ), as reported by Wagner ( 4 ), proposed the use of a dosing interval based on a systemic concentration multiple-dosing half-life ( t 1/2,md ) obtained by weighting individual half lives by their corresponding fractional area under curve following an intravenous bolus dose.

where n is the number of exponential terms (λi) and coefficients (Li) describing the systemic concentration time curve and f AUC,i is the fraction of the area under the curve related to each half-life.

For drugs, exhibiting multicompartment kinetics, the amount of drug in the body does not parallel systemic concentrations as is the case for a one compartment body model. Therefore, a different dosing interval must be chosen to describe the fall off of the amount of drug in the body during a dosage interval at steady-state. Such a multiple dosing half-life for amount of drug in the body ( t 1/2,A ) may also be defined:

Both of these multiple dosing half-lives can be defined in terms of well known pharmacokinetic parameters.

where k 10 is the elimination rate constant from the central compartment in a mammillary model where elimination occurs only from the central compartment; V 1 is the volume of the central compartment; CL is clearance; MRT is mean residence time in the body (following an intravenous bolus dose) and V ss is the volume of distribution at steady state. Note that t 1/2A can be determined by non-compartmental models, while t 1/2,md requires a fit of a polyexponential equation to the data to be able to determine V 1 .

Wagner ( 4 ) approached the problem in the context of mean residence time principles. He proposed that a suitable dosage interval can be estimated by a factor times the sum of mean transit time of the central compartment and the mean transit time of the absorption site. Such a dosing interval provides a ratio, the maximum steady-state plasma concentration/minimum steady-state plasma concentration, which averages ~2.

Veng-Pedersen and Modi ( 5 ) presented an explicit formula for simple calculations of the dosing time that results in a steady-state peak-to-trough ratio of 2 for extravascular administration described by a two-exponential expression ( i.e., one compartment body model with first order absorption). However, when more than two exponential terms are needed in the approximation of the drug level profile from the extravascular administration, iterative calculation of the dosing interval using a computer was suggested.

These various approximation equations recommended for estimation of dosing interval are summarized in Table I , where MRTc is the mean residence time in the central compartment and MIT is the mean input time.

Recommended Dosing Intervals

The problem has also been addressed by Kwan et al . ( 6 ) and Boxenbaum and Battle ( 7 ) who proposed the concept of an “effective half-life” (EHL) utilizing Wagner’s ( 8 ) drug accumulation index ( R c ):

where AUC ss ,0→τ is the area under the curve at steady-state (ss) during a dosing interval, τ, AUC sd ,0→τ is the area under the curve for a single dose (sd) during the same time interval, τ, i.e., from time 0 to time τ, while AUC sd ,0→∞ is the AUC for a single dose over all time, which for drugs following linear kinetics is equal to AUC ss ,0→τ . These authors then calculate the EHL from R c and τ assuming the drug follows one-compartment i.v. bolus dose kinetics at steady-state as per Eq. 7 :

Note in this relationship that the EHL is a function of the dosing interval, as opposed to being only a drug related value.

The EHL concept was developed by Kwan et al . ( 6 ) and Colburn ( 9 ) because they were unable to measure a relevant terminal half-life for NSAIDs that exhibited biliary cycling. Another half-life term, the “functional half-life” was introduced by Hsu et al . ( 10 ) to allow a simple half-life calculation at steady-state for a drug exhibiting nonlinear kinetics, ritonavir. This half-life has generally been designated by others as the “apparent multiple dosing half-life” ( t 1/2,app ), the terminology we use here. It is calculated using Eq. 8 at steady-state from the peak-to-trough ratios and the time interval (Δ t ) between the measured concentrations, which will always be less than the dosing interval (τ) for oral dosing:

Like the EHL, t 1/2,app will also be a function of the dosing interval, as opposed to being only a drug related value. The various half-life terms are defined in Table II in terms of the methods of determination. The last column in Table II provides comments on the usefulness of each term with respect to predicting drug accumulation upon multiple dosing as will be discussed subsequently.

Half-life Terms, Methods for Determining, Definitions and Comments

METHODS AND RESULTS

The literature was reviewed for drugs with reported terminal half lives longer than one day where for orally dosed drugs peak and trough measurements at steady state were available. An “apparent multiple dosing half life” ( t 1/2,app ) was determined using Eq. 8 . These results are presented in Table III . All of the apparent steady-state half-life values are less than 55% (15.3–54.3%) of the reported terminal half-lives.

Terminal Half-life vs Apparent Steady-state Half-life Calculated by Eq. 8

Because of the discrepancy between recommended dosing interval and terminal half-life for diazepam discussed above, we chose this drug for simulation. Dhillon and Richens ( 19 ) used a 2-compartment body model to describe the time course of a 10 mg i.v. diazepam dose in six healthy volunteers. The equation describing the concentration (C in mg/L)–time ( t in hours) data is:

In terms of Eq. 2 , where for diazepam L 1 =1207 mg/L, L 2 =183 mg/L, λ 1 =3.2 h −1 and λ 2 =0.0233 h −1 , it can be readily calculated that the fraction of AUC related to the terminal 29.7 h half-life (ln2/0.0233) accounts for the great majority (95.4%) of the area. We calculated the operational multiple dosing half-life ( t 1/2,op ) by iteration of Eq. 9 . The three values for each method (iteration, Wagner equation, Benet equation), the t 1/2,A calculated from AUMC/AUC and the EHL calculated at τ= t 1/2,op are given in Table IVA . Both the equations of Benet ( 3 ) and Wagner ( 4 ) underestimate the dosing interval that would result in a two fold increase in C max upon multiple dosing, which corresponded to a C max / C min ratio of 2.0 at steady-state. In contrast, the EHL and t 1/2,A markedly overestimate the operational multiple dosing half-life. Following intravenous dosing t 1/2,app will equal t 1/2,op when τ= t 1/2,op , but will change as a function of τ as noted in Table II and shown subsequently.

Calculations of Operational Half-lives (hr) at Steady-state Expected for Peak Multiple Dose to Peak Single Dose Ratios of 2.0 Compared to the Methods of Wagner ( 4 ), Benet ( 3 ), the t 1/2,A from MRT and the Effective Half-Life ( 6 , 7 ) when τ= t 1/2,op

However in most cases diazepam is given orally. The study of Kaplan et al. ( 14 ) included a 10 mg oral dose in four healthy volunteers yielding a mean absorption half-life of 0.36 h ( k a =1.92 h −1 ). We carried out simulations to determine the operational half-life using this absorption rate constant as given in Eq. 10 :

as well as for absorption half-lives of 0.25 h ( k a =2.77 h −1 ) and 2 h ( k a =0.347 h −1 ). The iterative operational half-life was determined when C max,ss / C max,sd =2.0. As opposed to the intravenous bolus dose, this operational half-life is close but does not correspond exactly to the maximum half-life calculated from the concentration at steady-state to the minimum concentration at steady-state, since t max changes upon multiple dosing. These results compared to the values predicted by Wagner’s and Benet’s equations, the t 1/2,A and the EHL when τ= t 1/2,op are given in Table IVB . Again the Wagner and Benet equations underestimate the iterative solutions, with the discrepancies markedly increasing as the absorption rate decreases. In contrast, the EHL and t 1/2,A are not very sensitive to k a , markedly overestimating the accumulation half-life for the rapid k a values, but as can be seen in Table IVB at some k a between 1.92 and 0.347 h −1 , each will equal t 1/2,op .

Simulations were carried out to elucidate the role of the various usual dosing intervals on the maximum to minimum plasma concentration ratio at steady-state, as well as the ratio of the maximum concentration at steady-state ( C max,ss ) to the maximum concentration for the single dose ( C max,sd ). The dosing schedules simulated were once a day (τ=24 h), twice a day (τ=12 h), three times a day (τ=8 h) or six times a day (τ=4 h). Time course for the simulations were set to 168 h for i.v. and 264 h for oral dosings and data generated every 0.125 h. All simulations were performed for 2-(oral and i.v.) compartment body models using WinNonlin (Version 2.1).

Figure 1 A depicts the ratio C max,ss / C min,ss and the ratio of maximum concentration at steady-state to the maximum concentration for a single dose ( C max,ss / C max,sd ) calculated for the dosing intervals 4 h, 8 h, 12 h and 24 h for the two-compartment i.v. doses of diazepam. Note that the ratio of peak to trough at steady-state increases with dosing interval while the inverse is seen with ratio of maximum concentrations at steady-state to that for the first dose. The curves intersect at the operational half-life, which gives a ratio of 2.0. Fig. 1B depicts these two ratios as a function of dosing interval for the oral data using the Kaplan et al . ( 14 ) reported k a =1.92. It is obvious that the ratios in Fig. 1 are close to 2.0 when the dosing interval is on either side of the operational half-life, but that marked deviations occur at more distant dosing intervals.

An external file that holds a picture, illustration, etc.
Object name is nihms438716f1.jpg

A The ratio ( black diamonds ) of maximum to minimum concentration at steady-state ( C max,ss / C min,ss ) and the ratio ( grey squares ) of maximum concentration at steady-state to the maximum concentration for a single dose ( C max,ss / C max,sd ) calculated for 4 h, 8 h, 12 h and 24 h dosing intervals for two-compartment i.v. doses of diazepam. B The ratio ( black diamonds ) of maximum to minimum concentration at steady-state ( C max,ss / C min,ss ) and the ratio ( grey squares ) of maximum concentration at steady-state to the maximum concentration ratio for a single dose ( C max,ss / C max,sd ) calculated for 4 h, 8 h, 12 h and 24 h dosing intervals for two-compartment oral data using k a value of 1.92 h −1 ( 14 ).

Selection of the appropriate dosing interval is an important clinical decision for a drug in a patient population. This is especially critical for narrow therapeutic index drugs. Concern may be expressed that the pharmacodynamic half-life will be more relevant than the pharmacokinetic value. We will return to this issue at the end of the Discussion section. However, if a pharmacokinetic half-life is clinically relevant and if it is to be used to design and justify an extended release drug product, then the appropriate half-life must be selected. Yet, many clinicians and clinical scientists believe and have been taught that the terminal half-life is the value most useful in selecting the dosing interval. However, we are unaware of any test of this hypothesis. In Table III we list eight drugs with long terminal half-lives, which we believe exhibit linear kinetics at the doses studied, where data for multiple oral dosing allowed determination of an apparent half-life at steady-state. In each case this apparent half-life was less than 55% of the reported terminal half-life. For rifabutin and sirolimus the apparent half-life values were 15–18% of the terminal half-lives.

Why does a clinician want to know the half-life? We believe that the most clinically relevant use of this value is the ability to predict the accumulation of drug in a patient upon multiple dosing. If a patient were dosed at intervals equivalent to this operational half-life, one would expect accumulation to double and peak to trough ratios at steady-state would be related by this half-life. In essence one is attempting to find a single half-life for a drug exhibiting multicompartment kinetics that at steady-state approximates what one would expect for a drug described by a 1-compartment body model. Benet ( 3 , 4 ) first addressed this problem in 1985 suggesting that 0.693 multiplied by the mean residence time in the central compartment (MRT c ) would predict the half-life. We used simulations using two compartment analyses of diazepam to evaluate the predictability. As can be seen in Table IVA the predictions using Benet’s approach for i.v. diazepam dosing underestimate the iteratively determined actual values. But the Benet predictions yield a much better estimate than the terminal half-life, which is 29.7 h from the 2-compartment data fit and the EHL and t 1/2,A . Wagner, in referencing the Benet equations, realized that the values were underestimated and added a scaling factor, which here for diazepam does yield better, but still underestimated predictions. In contrast, the EHL and t 1/2,A markedly overestimate the operational half-life. From the 5.30 h operational half-life given in Table IVA one can understand the proposed recommended diazepam dosing of three to four times a day, although the source of this labeling recommendation is unknown to us.

The most surprising finding in this work is the sensitivity of the operational half-life to the absorption rate constant following oral dosing. For example, even for a very rapid absorption ( t 1/2 =0.25 h) the operational half-life following oral dosing increases by 144% from that found for i.v. dosing (compare values for k a =2.77 h −1 in Table IVB with values in IVA). Note that for an absorption half-life of 2 h ( k a =0.347 h −1 ) in the two-compartment model, the operational half-life (35.0 h) is larger than the terminal half-life (29.7 h). This sensitivity of the operational half-life to the absorption rate constant probably explains the frequently reported differences in peak to trough ratios noted between morning and evening dosing for twice daily drug regimens. One would expect differences in absorption rate for the dose taken at bed time versus that taken in the morning. It also explains why peak to trough ratio across a population and even within an individual are more variable than average concentrations, since the former is dependent on k a and the latter on clearance.

Reviewers of drafts of this manuscript were concerned that diazepam may be unique and that other drugs may not exhibit this marked change in t 1/2,op with relatively small changes in the absorption rate constant. In a following manuscript we will show that two further drugs in Table III , everolimus and bepridil, also show significant changes in t 1/2,op as a function of k a .

The relationships depicted in Fig. 1 provide some useful insights. At dosing intervals greater than t 1/2,op calculating a half-life from C max,ss / C min,ss will overestimate t 1/2,op , while calculating a half-life from C max,ss /C max,sd will underestimate t 1/2,op . At dosing intervals less than t 1/2,op, the opposite will occur. Looking back at Table III we can thus suggest that the apparent steady-state half-lives for all drugs, except chloroquine and chlorthalidone, overestimate t 1/2,op , and chlorthalidone is well estimated, since t 1/2,op approximates the dosing interval. The value for sirolimus, in this study where the patients also received concomitant cyclosporine, is probably also well estimated. That the value for diazepam is overestimated in Table III is confirmed since the calculated t 1/2,op for diazepam using the pharmacokinetic parameters from that study can be seen in Table IV .

Reviewing the recent literature provides a confirming example for the principles presented here. Iwamoto et al . ( 20 ) evaluated the accumulation of the HIV integrase inhibitor raltegravir following single and multiple twice daily 100, 200, 400, 600 and 800 mg oral doses. The average C max,ss / C max,sd was 1.0, indicating no accumulation. They also compared areas under the curve at steady-state to areas under the curve for the same time interval (12 h) for the first dose and reported an average accumulation of 1.06 (This ratio substituted into Eq. 7 indicates an effective half-life of 2.90 h). Yet, Iwamoto et al . ( 20 ) report mean terminal half-lives for the 5 different doses that range from 9.9 to 12.1 h. It is obvious that the t 1/2,op for raltegravir must be significantly shorter if no accumulation is observed, as here also shown for the EHL calculation.

However, an even more potentially useful application is apparent from the results in Table IVB . Today, the general belief in the development of extended release dosage forms is that drug release from the dosage form must be the rate controlling step. This is often difficult to achieve because of gastrointestinal transit time and poor absorption from the ileum and colon. Yet, the results in Table IVB suggest, that at least for some drugs, a relatively modest change in the absorption rate (by altering drug release from the dosage form) can markedly change the operational half-life. Drug product formulators avoid changing a drug’s disposition kinetics, since this would involve inhibiting, inducing or activating enzymes and transporters that certainly would generate regulatory concerns; furthermore as mentioned above creating drug delivery devices that control the rate of oral absorption and make it the rate limiting step is difficult to achieve and when achieved frequently lead to patent protection that precludes others from using a similar approach. However, making relatively small changes in absorption are easily within the expertise of drug product formulators, and because such changes have frequently been employed in the past, such as adding a shellac coating to micro/nano-particles, little patent protection would be afforded. Thus, we now recognize that for drugs where single dose kinetics has been accurately quantitated, simulations may be easily carried out where the absorption rate is modified to determine the sensitivity of the multiple dosing operational half-life to these changes as we have shown for diazepam in Table IVB . The procedure is as follows:

  • A pharmacokinetic compartment model is fit to the i.v. data, or more likely the oral data, in humans yielding equations such as Eqs. 9 and 10 for diazepam. Test for linearity by evaluating different doses or multiple dosing.
  • Calculate the pharmacokinetic parameters that describe the single dose data. If only oral dosing data is available, distinguish the absorption rate constant from the disposition parameters by deliberately altering absorption rate as described by Boni et al . ( 21 ) or using an intercept method ( 22 ).
  • Choose the desired dosing interval, τ.
  • Modify the equation describing single dose oral data to a multiple dosing equation at steady-state by multiplying each exponential function by the appropriate multiple dosing function, 1/[1−exp(−λ i × τ)], and changing the time function in each exponential term to t’, the time within a dosing interval, as first presented by Dost ( 23 ) and implemented by Benet ( 24 ) for his general treatment of mammillary models. For Eq. 10 , with a k a = 1.92 h −1 , this would result in the following Eq. 11 : C oral = − 1810 1 − e − 3.2 τ e − 3.2 t ′ + 185 1 − e − 0.0233 τ e − 0.0233 t ′ + 1625 1 − e − k a τ e − k a t ′ (11)
  • Iterate with different values of k a to determine which k a value will achieve an operational half-life that equals the desired τ. Note that all of the coefficients in Eqs. 10 and 11 contain k a in the numerator and denominator ( 24 ). This step will always be successful, but for certain drugs and selected dosing intervals the solution may indicate that only a dosage form where absorption is the slowest, rate controlling step, will succeed. Then perhaps a shorter τ may be tested. However, for many drugs we believe that readily achievable absorption half-lives, that are less than the terminal half-life, will suggest that extended release formulations will be successful.

What are the take home messages from this analysis?

  • Predicting a dosing interval for intravenous bolus dosing accumulation at steady-state can be reasonably obtained by 0.693MRTc, although this will be an underestimate. The terminal half-life will be a marked over prediction of the appropriate dosing interval.
  • Even for drugs where the terminal half-life represents the great majority of the AUC (e.g., diazepam >95%), t 1/2,op may be much smaller for both i.v. and oral dosing.
  • Following oral dosing, even for very fast absorption, t 1/2,op may be significantly greater than that for i.v. dosing. The t 1/2,op value may be quite sensitive to changes in k a . In some cases, with slow absorption, t 1/2,op can be greater than the terminal half-life. But this is not restricted to flip-flop models where k a <λ z (where λ z is the terminal disposition constant following i.v. dosing). Here for diazepam in Table IVB , t 1/2,op > t 1/2,z when the absorption half life was only 2 h vs t 1/2,z =29 h and t 1/2,op =35 h
  • For orally dosed drugs, it is not possible to simply predict t 1/2,op ; this can only be done by iterative calculations, as first suggested by Veng-Pedersen and Modi ( 5 ).
  • The terminal half-life only describes drug loss from the body after drug dosing has stopped; for a number of drugs, it is not a good predictor of accumulation at steady-state, or of the time course of drug fall off during a dosing interval at steady-state, even when the terminal half-life relates to the majority of the AUC.
  • The difficulty pointed out here for predicting accumulation is most significant for drugs exhibiting long terminal half-lives as given in Table III . Frequently, for drugs with half-lives of 12 h or less, in our experience, accumulation prediction error will not be significant. However, this cannot be assumed and must be evaluated. For example, see the raltegravir discussion above where accumulation is not predicted from the terminal half-life.
  • The findings in this paper provide a road map for drug formulators to predict the changes in drug absorption needed to yield an extended release dosage form with optimal accumulation and peak to trough ratios at steady-state. At least for some drugs, as demonstrated here, absorption need not be the rate limiting process.

Thanks to a perceptive reviewer who suggests that the sensitivity of t 1/2,op to k a may be the result of the marked difference between the terminal half life of diazepam (29.7 h) and its short t 1/2,md (4.10 h, Table IV ), since t 1/2,md =0.693/ k 10 ( Eq. 4 ). Whether this marked difference is diagnostic of drugs amenable to simple formulation of extended release products as described above should be investigated.

Recently there has been a great deal of discussion concerning the ambiguity of the half-life definitions addressed in this manuscript, which can be found on the PharmPK discussion website [ http://www.boomer.org/pkin/ ] under the 2008 discussion sections titled “Terminal half-life and elimination half-life” and “What is ‘apparent’ half-life?” Further discussions on the web site relate to PK–PD relationships under the topic section “Does half-life in blood inform about drug targets?” There are many drugs where the time course of clinical response does not relate to the pharmacokinetic half-life, such as warfarin, insulin, levodopa, prednisolone, simvastatin, fluoxetine, epoetin and omeprazole among others. However, we contend that under steady-state dosing conditions this lack of a direct relationship between systemic concentrations and effect site concentrations (or measures of clinical response) following a single dose are immaterial if the clinical response is related to systemic exposure.

Another manifestation of the concern about PK/PD discontinuities is found in the anesthesia literature where “context sensitive half-times” are defined ( 25 ). The context-sensitive half-time is the time required for systemic concentrations of a drug to decrease by 50% after discontinuation of drug administration, a value that is a function of the duration of drug administration. Although these anesthesia studies are primarily related to continuous infusions of drug, rather than repeated multiple dose administration, the analysis presented here for intravenous diazepam ( Table IVA ) is consistent with finding a 50% fall off time that differs significantly from the terminal half-life.

The lack of continuity of half-life between a pharmacokinetic parameter and a pharmacodynamic effect following a single dose is not the subject of this analysis. A dosage regimen for an i.v. bolus formulation or an immediate release oral formulation will be recommended based on safety and efficacy studies. Daily dose adjustments will result from significant changes in clearance and/or bioavailability. However, dosing interval decisions in many cases will be based on a consideration of half-life. In our experience, clinicians are concerned, especially for narrow therapeutic index drugs, about potential toxicities resulting from peak concentrations that are too high and lack of efficacy for trough concentrations that are too low. Those concerns are addressed by the relationship between dosing interval and the operational half-life. When disease states, age, sex, genetic polymorphisms or alternate dosing formulations change clearance divided by bioavailability, the dosing rate can be appropriately modified to maintain that desired exposure, while calculation of the appropriate t 1/2,op under these conditions can be used to adjust the dosing interval.

CONCLUSIONS

Half-life is the oldest, but the least well understood pharmacokinetic parameter. The concepts that we have learned about half-life only hold for drugs that following i.v. dosing are best fit by a one-compartment body model, which is true for a very limited number of compounds, if any. Many different half-life terms have been introduced in an attempt to simplify multicompartment kinetics in terms of a single value that would be useful in predicting accumulation at steady-state. These various half-lives have been reviewed here and summarized in Table II . To predict drug accumulation upon multiple dosing and peak to trough ratios at steady-state for a particular dosing interval one must determine an operational half-life. For intravenous multiple bolus doses this operational half-life may be approximated by adding a small increment to a half-life related to the mean residence time in the central compartment. For oral multiple doses the operational half-life may only be determined by iterative computer analyses. In addition, this operational half-life may be very sensitive to changes in the absorption rate, and thus drug formulators can use the methodology presented here as a road map in the development of extended release dosage forms.

ACKNOWLEDGEMENTS

Drs. Sahin and Benet were supported in part during the course of this work by NIH Grant R21 GM75900. The authors appreciate the critical reviews of this work as it progressed and the suggestions of Drs. Malcolm Rowland, Nicholas Holford, Harold Boxenbaum, Svein Øie and Stephen Hwang. Thanks also to Ms. Anita Grover for sharing her preliminary evaluations of everolimus and bepridil simulations with changing absorption rate.

ABBREVIATIONS

Nursing Pharmacology Chapter 2:  General Principles:  Pharmacokinetics

Drug Accumulation

Bioavailability

Medical Information

Half-Life: Exploring the Significance and Applications in Pharmacology and Radioactive Decay

Half-Life: Exploring the Significance and Applications in Pharmacology and Radioactive Decay

Introduction:.

Half-life is a fundamental concept used in pharmacology and the study of radioactive decay. It refers to the time it takes for half of a substance to decay or undergo a specific process. This comprehensive article aims to provide an in-depth understanding of half-life, its significance, calculations, and its applications in pharmacokinetics and radioactive decay.

Half-Life in Pharmacology:

In pharmacology, half-life plays a crucial role in determining the dosage regimen and duration of action of drugs. Key points related to half-life in pharmacology include:

  • Definition: The half-life of a drug refers to the time it takes for the concentration of the drug in the body to decrease by half.
  • Drug Elimination: The half-life helps determine the rate at which a drug is eliminated from the body, influencing factors such as dosing frequency and therapeutic efficacy.
  • Steady State: It takes approximately five half-lives for a drug to reach steady-state concentration, where the rate of drug administration is equal to the rate of elimination.

Calculating Half-Life:

The half-life of a substance can be determined using mathematical calculations based on its elimination rate constant (k) or clearance (CL). The half-life (t1/2) can be calculated using the formula: t1/2 = 0.693 / k. The half-life calculation is vital for determining appropriate drug dosing intervals and achieving optimal therapeutic levels.

Half-Life in Radioactive Decay:

Half-life is also a critical concept in the study of radioactive decay. Key points related to half-life in radioactive decay include:

  • Definition: The half-life of a radioactive substance is the time it takes for half of the radioactive nuclei to undergo decay and transform into other elements or isotopes.
  • Decay Constant: The decay constant (λ) represents the probability of decay per unit time, and it is related to the half-life through the equation: λ = 0.693 / t1/2.
  • Radioactive Dating: By measuring the ratio of radioactive isotopes to their decay products, scientists can estimate the age of objects and materials through the determination of their half-lives.

Applications of Half-Life:

The concept of half-life finds practical applications in various fields:

  • Drug Dosage Regimens: Understanding the half-life of a drug helps establish appropriate dosing intervals to maintain therapeutic levels in the body.
  • Radioactive Safety and Nuclear Medicine: Half-life determines the duration of radioactivity and guides decisions related to radiation protection and the use of radioactive materials in diagnostic imaging and cancer treatments.
  • Carbon Dating and Archaeology: By measuring the half-life of carbon-14 isotopes, scientists can determine the age of organic materials and archaeological artifacts.

Conclusion:

Half-life is a fundamental concept in pharmacology and the study of radioactive decay. By understanding its significance, calculations, and applications in pharmacokinetics and radioactive decay, scientists, healthcare professionals, and researchers can make informed decisions regarding drug dosing, radiation safety, and archaeological dating.

Hashtags: #HalfLife #Pharmacology #RadioactiveDecay #DrugDosage #RadiationSafety

On the Article

Krish Tangella MD, MBA picture

Krish Tangella MD, MBA

Alexander Enabnit picture

Alexander Enabnit

Alexandra Warren picture

Alexandra Warren

Please log in to post a comment.

Related Articles

Test your knowledge, asked by users, related centers, related specialties, related physicians, related procedures, related resources, join dovehubs.

and connect with fellow professionals

Related Directories

At DoveMed, our utmost priority is your well-being. We are an online medical resource dedicated to providing you with accurate and up-to-date information on a wide range of medical topics. But we're more than just an information hub - we genuinely care about your health journey. That's why we offer a variety of products tailored for both healthcare consumers and professionals, because we believe in empowering everyone involved in the care process. Our mission is to create a user-friendly healthcare technology portal that helps you make better decisions about your overall health and well-being. We understand that navigating the complexities of healthcare can be overwhelming, so we strive to be a reliable and compassionate companion on your path to wellness. As an impartial and trusted online resource, we connect healthcare seekers, physicians, and hospitals in a marketplace that promotes a higher quality, easy-to-use healthcare experience. You can trust that our content is unbiased and impartial, as it is trusted by physicians, researchers, and university professors around the globe. Importantly, we are not influenced or owned by any pharmaceutical, medical, or media companies. At DoveMed, we are a group of passionate individuals who deeply care about improving health and wellness for people everywhere. Your well-being is at the heart of everything we do.

For Patients

For professionals, for partners.

University of Nottingham

School of Health Sciences

Pharmacology: Half-life of Drugs

Narration audio (MP4)

Narration audio (OGG)

Toggle Navigation

Half-life of Drugs

Experimentally the half life can be determined by giving a single dose, usually intravenously, and then the concentration of the drug in the plasma is measured at regular intervals. The concentration of the drug will reach a peak value in the plasma and will then fall as the drug is broken down and cleared from the blood.

The time taken for the plasma concentration to halve is the half life of that drug. Some drugs like ibuprofen have very short half lives, others like warfarin and digoxin, take much longer to eliminate from the plasma resulting in a long half life. So drugs like ibuprofen that are cleared from the blood more rapidly than others need to be given in regular doses to build up and maintain a high enough concentration in the blood to be therapeutically effective.

Your browser does not support video playback

  • Download the Half-life of Drugs video (MP4)
  • Download the Half-life of Drugs video (WEBM)

how to do half life problems pharmacology

Advertisement

Half-Life Formula: Components and Applications

  • Share Content on Facebook
  • Share Content on LinkedIn
  • Share Content on Flipboard
  • Share Content on Reddit
  • Share Content via Email

Man in lab coat measuring a decayed skull in a white laboratory

In nuclear physics, the concept of half-life plays a crucial role in understanding the decay of radioactive substances. Scientists use the half-life formula in other disciplines to predict the rate of decay, as well as measure the age of ancient artifacts through carbon dating.

What Is Half-life?

Components of the half-life formula, applications of the half-life formula.

In the context of radioactive decay and nuclear physics, half-life describes the time it takes for half of a quantity of a substance undergoing decay to go through transformation. In simpler terms, half-life is the duration it takes for a radioactive substance to lose half of its initial radioactivity.

For example, if you start with a certain amount of a radioactive substance, after one half-life, half of that substance will have decayed, and you will have half of the original amount. After two half-lives, three-quarters will have decayed, and so on.

Radioactive Decay and Isotopes

Half-life is a characteristic property of each radioactive isotope , and it plays a crucial role in understanding the stability and decay of atomic nuclei. You can express the concept mathematically through an exponential decay model, where the rate of decay is proportional to the remaining quantity of the substance.

The half-life of a radioactive isotope — denoted by T 1/2 — varies widely depending on the specific isotope . Each has its own unique half-life. Some isotopes have very short half-lives, measured in seconds or minutes, while others have half-lives that extend over thousands or millions of years.

The concept of half-life is not limited to radioactive decay; other fields like medicine, chemistry and environmental science also measure half-life.

The half-life formula is:

Here are the formula's different components:

  • N(t) represents the remaining quantity of the radioactive substance at time t .
  • N is the initial quantity of the substance at time t = 0 . The initial quantity sets the starting point.
  • e is the base of the natural logarithm.
  • λ is the decay constant, a measure of the rate of decay for the radioactive isotope.

In the formula, e –λt is the core exponential decay factor, governing the decrease in quantity over time. As time ( t ) increases, this factor approaches 0, indicating an exponentially decaying quantity due to radioactive decay.

You can find a half-life calculator online to simplify the process of solving half-life problems.

Here are a few uses for the half-life formula.

  • Radioactive decay : Scientists use the half-life formula to describe the decay process of radioactive isotopes. It helps determine the rate of decay and predict how much of a substance will remain after a certain period.
  • Carbon dating : Carbon dating relies on the half-life of carbon-14 ( 14 C) to estimate the age of organic materials. By measuring the ratio of carbon-14 to carbon-12 in a sample, scientists can calculate how many half-lives have elapsed since the organism died.
  • Archaeology and geology : Half-life calculations are essential in dating ancient artifacts and geological samples. For example, scientists can use the decay of uranium to lead to determine the age of rocks.
  • Medical imaging : Radioactive substances used in medical imaging have known half-lives. Understanding the half-life allows medical professionals to determine the appropriate dosage and timing for imaging procedures.

This article was created in conjunction with AI technology, then was fact-checked and edited by a HowStuffWorks editor.

Please copy/paste the following text to properly cite this HowStuffWorks.com article:

IMAGES

  1. Pharmacology Glossary: Half-Life

    how to do half life problems pharmacology

  2. Half Life (Drug Calculations Practice Problems

    how to do half life problems pharmacology

  3. Half Life of Drug| Methods to determine Half Life| Biopharmaceutics

    how to do half life problems pharmacology

  4. How To Calculate Half Life Of A Drug?

    how to do half life problems pharmacology

  5. PPT

    how to do half life problems pharmacology

  6. PPT

    how to do half life problems pharmacology

VIDEO

  1. TRICKS ON HALF LIFE PROBLEMS

  2. 7- Basics of pharmacology: Pharmacokinetic: Execration and Half Life

  3. Pharmacokinetic I

  4. HLS Pharmacology : Lipids Lowering agents / Lecture 1

  5. Half-Life w dodatku do Half-Life #shorts #halflife

  6. Solving half-life problems with a fractional number of half lives

COMMENTS

  1. Drug Half-life Explained: Calculator, Variables & Examples

    The half-life of a drug is an estimate of the time it takes for the concentration or amount in the body of that drug to be reduced by exactly one-half (50%). The symbol for half-life is t½. For example, if 100mg of a drug with a half-life of 60 minutes is taken, the following is estimated: 60 minutes after administration, 50mg remains

  2. Half Life

    Issues of Concern Half-life is one of the oldest pharmacokinetic parameters discussed in the medical community yet continues to be a source of confusion for many medical students and even well-versed clinicians. [6] For this reason, the USMLE examiners continue to evaluate medical students and licensed physicians on this elusive topic.

  3. Pharmacology Drug Half Life Practice Questions Flashcards

    5. Study with Quizlet and memorize flashcards containing terms like Drug X has a half-life of 8 hours. If 800mg is administered at 1:00 a.m, how much of the drug would be eliminated after 24 hours?, Drug A has a half-life of 4 hours. If 600mg is administered at 8:00 p.m, how much of the drug would be eliminated after 24 hours, Drug V has a half ...

  4. Applied Pharmacology 4, Half Life of Drugs

    Understanding half life is important to inform how and when we administer any medication. These are not casual viewing, but are for serious students who want...

  5. Kaplan USMLE Step 1: Calculating a drug's half-life

    The half-life (t 1/2) is the time it takes for the plasma concentration of a drug or the amount of drug in the body to be reduced by 50%. 50% will remain after one half-life, 25% after two half-lives, 12.5% after three half-lives, and 6.25% after four half-lives. For more prep questions on USMLE Steps 1, 2 and 3, view other posts in this series.

  6. Pharmacokinetics

    Pharmacokinetics. Images. Volume of Distribution (Vd) Volume of distribution (Vd) relates amount of drug in body to plasma concentration. Vd = (amount of drug in the body) / (plasma drug concentration) Vd is changed in disease states that decrease plasma proteins. a decrease in plasma proteins decreases binding of drug to plasma proteins.

  7. Kaplan USMLE Step 1 prep: What's half-life of investigational drug?

    The half-life determines the rate at which a drug concentration rises during a constant infusion and also the rate at which the concentration falls after drug administration is stopped. It is commonly accepted that it takes four to five half-lives to reach steady state, as shown in the figure to the right.

  8. Drug Half-life

    Pharmacology and drug half-life or half life. View my other pharmacology videos below: (1) Pharmacokinetics & ADME: http://youtu.be/CMRZqdrkCZwDRUG ABSORPTIO...

  9. The Operational Multiple Dosing Half-life: A Key to Defining Drug

    Half-life (t 1/2) is the oldest but least well understood pharmacokinetic parameter, because most definitions are related to hypothetical 1-compartment body models that don't describe most drugs in humans.Alternatively, terminal half-life (t 1/2,z) is utilized as the single defining t 1/2 for most drugs.However, accumulation at steady state may be markedly over predicted utilizing t 1/2, z.

  10. RLO: Pharmacology: Half-life of Drugs

    Introduction. The duration of action of a drug is known as its half life. This is the period of time required for the concentration or amount of drug in the body to be reduced by one-half. We usually consider the half life of a drug in relation to the amount of the drug in plasma. A drug's plasma half-life depends on how quickly the drug is ...

  11. Half-Life in Pharmacology

    Steady State Importance of Half-Life in Medicine Issues with Half-Life and Medication Lesson Summary Frequently Asked Questions What does drug half-life mean? The half-life of a drug is...

  12. Relevance of Half-Life in Drug Design

    Plasma Abstract Drug half-life has important implications for dosing regimen and peak-to-trough ratio at the steady state. A half-life of 12-48 h is generally ideal for once daily dosing of oral drugs.

  13. Half-life

    In brief : Half-life (t½) is the time required to reduce the concentration of a drug by half. The formula for half-life is (t½ = 0.693 × Vd /CL) Volume of distribution (Vd) and clearance (CL) are required to calculate this variable.

  14. Half Life (Drug Calculations Practice Problems

    Share 2.8K views 2 years ago Drug Calculations: Practice Problems Questions we discussed are: 00:00 - A new developed medication ..... drug eliminated by first-order kinetics. From the pr ...more...

  15. Pharmacokinetics for Nursing Pharmacology: Drug Half-Life

    Introduction Half-life: (t 1/2 ), the time required to decrease the amount of drug in body by 1/2 during elimination (or during a constant infusion). Assumption: Single body compartment size = volume of distribution (V d) Blood or plasma considered in equilibrium with total volume of distribution

  16. Half-Life: Exploring the Significance and Applications in Pharmacology

    Definition: The half-life of a radioactive substance is the time it takes for half of the radioactive nuclei to undergo decay and transform into other elements or isotopes. Decay Constant: The decay constant (λ) represents the probability of decay per unit time, and it is related to the half-life through the equation: λ = 0.693 / t1/2.

  17. Half Life (t1/2) of a Drug

    The half-life of a drug is an estimate of how long it takes for the concentration or amount of that drug in the body to be reduced by exactly one half (50%)....

  18. RLO: Pharmacology: Half-life of Drugs

    Half-life of Drugs. Experimentally the half life can be determined by giving a single dose, usually intravenously, and then the concentration of the drug in the plasma is measured at regular intervals. The concentration of the drug will reach a peak value in the plasma and will then fall as the drug is broken down and cleared from the blood.

  19. Quiz & Worksheet

    Quiz & Worksheet Goals. In this worksheet and quiz you'll assess your understanding of: What half-life means in regards to medication. How you describe the process of removing medication from the ...

  20. Drug Half Life & Dosing Medications

    How do we figure out how often to give each drug? A quick example of how a drug's half-life influences drug dosing frequencyCheck out last week's Reinhartz ...

  21. Half-Life Formula: Components and Applications

    Components of the Half-life Formula. The half-life formula is: N (t) = N e-λt. Here are the formula's different components: N (t) represents the remaining quantity of the radioactive substance at time t. N is the initial quantity of the substance at time t = 0. The initial quantity sets the starting point. e is the base of the natural logarithm.

  22. Pharmacology Drug Half Life Practice Questions Flashcards

    5. Study with Quizlet and memorize flashcards containing terms like Drug X has a half-life of 8 hours. If 800mg is administered at 1:00 a.m, how much of the drug would be eliminated after 24 hours?, Drug A has a half-life of 4 hours. If 600mg is administered at 8:00 p.m, how much of the drug would be eliminated after 24 hours, Drug V has a half ...

  23. Half Life (T 1/2)

    https://usmleqa.com/http://usmlefasttrack.com/?p=5020 Half, Life, (T, 1/2), -, Pharmacology, Findings, symptoms, findings, causes, mnemonics, review, what ...

  24. half life calculations

    The video demonstrates how to set up a table used for solving half-life problems.