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The elimination half-life of a drug is a pharmacokinetic parameter that is defined as the time it takes for the concentration of the drug in the plasma or the total amount in the body to be reduced by 50%. In other words, after one half-life, the concentration of the drug in the body will be half of the starting dose. With each additional half-life, proportionately less of the drug is eliminated. However, the time required for the drug to reach half of the original concentration remains constant. In general, the effect of the drug is considered to have a negligible therapeutic effect after 4 half-lives, that is, when only 6.25% of the original dose remains in the body. Clearance & Half-Life - The Pharmacokinetics SeriesPlay Illustrative exampleIn the event that a 100 milligram (mg) dose of an intravenous drug with a half-life of 15 minutes is administered, the following would be true:
Related termsThere are several other terms that are closely related to the drug half-life, including:
Clearance and volume of distributionDrug Discovery eBook Compilation of the top interviews, articles, and news in the last year. There are two factors that affect the elimination half-life of a drug, which include its clearance and volume of distribution. The clearance of the drug (CL) refers to the rate at which the body eliminates the drug from the body. Alternatively, the volume of distribution (Vd) refers to the distribution of the drug around the body. The relationship between CL and Vd is as follows: t(½) = ln(2)/ λ = ln(2)*(Vd/CL) = 0.693*(Vd/CL) The elimination half-life is considered to be constant and independent of the concentration of the drug in the body. Clinical usesThe elimination half-life is a useful pharmacokinetic parameter, as it provides an accurate indication of the length of time that the effect of the drug persists in an individual. Moreover, the elimination half-life can also show if accumulation of the drug is likely to occur with a multiple dosing regimen. This is helpful when it comes to deciding the appropriate dose and frequency of a prescribed drug. Along with other pharmacokinetic data and values about the individual patient, the half-life can help health practitioners to estimate the rate at which a drug will be eliminated from the body, as well as how much will remain after a given time period. From this information, appropriate decisions can be made on how to promote patient health outcomes. As with previous half-lives, this indicates that elimination is 50% complete after one elimination half-life, 75% complete after two elimination half-lives, and about 99% complete after seven elimination half-lives. Since elimination represents the final removal of drug molecules from the body, the elimination half-life also serves as the determining factor of how much of the originally absorbed drug remains in the body, so that 50% of the absorbed drug remains after one elimination half-life, 25% remains after two elimination half-lives, and only about 1% remains after seven elimination half-lives. The elimination half-life is the half-life value reported in drug handbooks as an indication of how long a drug remains active in the body. View chapterPurchase book Read full chapter URL: https://www.sciencedirect.com/science/article/pii/B9780123695215000105 Principles of Drug Disposition in the Critically Ill ChildJeffrey L. Blumer, in Pediatric Critical Care (Fourth Edition), 2011 Half-LifeElimination half-life of a drug is a hybrid term that is a function of both clearance and volume of distribution: t1/2ß=0.693undefinedKe=0.693VdCl This is the pharmacokinetic parameter most commonly used by clinicians, but it is often misconstrued to signify drug elimination. However, as shown in the equation, either a change in Cl or a change in Vd can result in a change in t1/2β. The therapeutic implications of these alterations are clearly different. It is also apparent that if pathophysiologic changes result in offsetting changes in Vd and Cl, the elimination half-life could remain unaffected in the face of significant disease. The most important clinical application of half-life is as a determinant of drug dosing. Four to five half-lives are required for a drug to reach steady-state plasma concentration at any given dose. This is true whether therapy is being initiated or the dose is being changed. View chapterPurchase book Read full chapter URL: https://www.sciencedirect.com/science/article/pii/B9780323073073101168 ZIn Mosby's Dental Drug Reference (Eleventh Edition), 2014 Drug Class:Analgesic Mechanism of ActionA synthetic peptide that selectively binds to and blocks N-type voltage-sensitive calcium channels located on afferent nerves in the spinal cord. Therapeutic Effect: Blocks excitatory neurotransmitter release, reducing sensitivity to painful stimuli. UsesReduction of chronic pain in the body PharmacokineticsElimination Half-life: 4.6 hr after intrathecal administration. 50% bound to plasma proteins; metabolized in multiple organs. Excreted in urine as proteolytic degradation products. Indications and Dosages▸ Pain ControlInitially, 2.4 mcg/day (0.1 mcg/hr). May titrate to maximum of 19.2 mcg/day (0.8 mcg/hr). Side Effects/Adverse ReactionsFrequentDizziness, nausea, somnolence, weakness, diarrhea, confusion, ataxia, headache, vomiting, gait disturbance, memory impairment, hypertonia OccasionalAnorexia, visual disturbances, anxiety, urinary retention, speech disorder, aphasia, nystagmus, paresthesia, fever, hallucinations, nervousness, vertigo RareInsomnia, dry skin, constipation, arthralgia, myalgia, tremor Precautions and ContraindicationsHistory of psychosis, presence of infection at the injection site, uncontrolled bleeding, or spinal canal obstruction that impairs CSF circulation, IV administration Drug Interactions of Concern to Dentistry• Enhanced CNS depression: all CNS depressants Serious Reactions! Atrial fibrillation, cerebral vascular accident, seizures, kidney failure (acute), myoclonus, and psychosis occur rarely. Dental ConsiderationsGeneral:• Determine why patient is taking the drug. •For use in the hospital setting. Consultations:• Medical consultation may be required to assess disease control and patient's ability to tolerate stress. Teach Patient/Family to:• Encourage effective oral hygiene to prevent soft tissue inflammation. •Update health and medication history if physician makes any changes in evaluation or drug regimens; include OTC, herbal, and nonherbal remedies in the update. View chapterPurchase book Read full chapter URL: https://www.sciencedirect.com/science/article/pii/B9780323169165000249 Basic pharmacokineticsWilliam Clarke, Mark A. Marzinke, in Contemporary Practice in Clinical Chemistry (Fourth Edition), 2020 Elimination half-lifeThe elimination half-life is the amount of time required for 50% of the drug to be removed from the blood during the terminal elimination phase. This can be determined graphically from a linear plot of time versus log concentration (Fig. 50.4), or it can be calculated using the equation below: Figure 50.4. Graphical determination of half-life (t1/2). t1/2=0.693kel In addition, based on the elimination half-life, the time needed for clearance of a drug can be estimated; approximately, 97% of the drug is eliminated by five half-lives, and just over 99% of the drug is eliminated when seven half-lives have passed. View chapterPurchase book Read full chapter URL: https://www.sciencedirect.com/science/article/pii/B9780128154991000508 Pharmacokinetic and Toxicokinetic ModelingR.W. Seabury, C.M. Stork, in Encyclopedia of Toxicology (Third Edition), 2014 Elimination Half-Life (t1/2)Elimination half-life is the time required to produce a 50% reduction in blood or plasma concentration. This value is estimated using the following equation: [9]t1/2=0.693×VDCl Since the first-order elimination rate constants ke and β can be calculated by dividing VD by Cl, the half-life of a xenobiotic that follows a one- or two-compartment model can be calculated as follows: (1) one-compartment model – t1/2 = 0.693/ke and (2) two-compartment model – t1/2 = 0.693/β. These values should remain relatively consistent in xenobiotics following these models. Conversely, the half-lives of xenobiotics undergoing zero-order elimination, such as overdoses with aspirin and acetaminophen, cannot be calculated using eqn [9] because the time required to produce a 50% reduction in blood/plasma concentration is variable (i.e., elimination is not proportionally related to concentration). View chapterPurchase book Read full chapter URL: https://www.sciencedirect.com/science/article/pii/B9780123864543007661 Physiological Aspects Determining the Pharmacokinetic Properties of DrugsKoen Boussery, ... Johan Van de Voorde, in The Practice of Medicinal Chemistry (Fourth Edition), 2008 D Elimination Half-Life (T1/2)The elimination half-life (T1/2) is the time it takes for the elimination processes to reduce the plasma concentration or the amount of drug in the body by 50 percent. Elimination half-life is a composite pharmacokinetic parameter determined by both clearance and volume of distribution (Vd), as described by the following equation: (23.8)T1/2=0.7VdCl Elimination half-life is increased by an increase in volume of distribution or a decrease in clearance, and vice versa. This is because a decrease in the efficiency of elimination (and therefore in clearance) would, of course, cause an increase in the time needed to reduce the plasma concentration by 50 percent. On the other hand, the larger the volume of distribution, the more the drug is concentrated in the tissues rather than in the plasma. It is, however, the drug in plasma that is exposed to the elimination mechanisms. Therefore, an increase in volume of distribution also increases elimination half-life. For the simplest cases, elimination half-life may be used to make decisions about drug dosage, and can be derived from the plasma concentration–time profile as the time it takes for a random plasma concentration in the elimination phase to be halved (see Figure 23.7). It does not matter at what concentration half-life is measured, as long as it is measured in the mono-exponential elimination phase of the curve. Therefore, the time for the plasma concentration to drop from 10 to 5 mg/L is the same as from 8 to 4 mg/L or from 2 to 1 mg/L. It becomes more complicated when the plasma concentration follows a multiexponential pattern of decline and two or more half-lives can be calculated. This situation is left out of the discussion in this chapter, but the interested reader can refer to the textbooks on pharmacokinetics mentioned at the end of the chapter. View chapterPurchase book Read full chapter URL: https://www.sciencedirect.com/science/article/pii/B9780124172050000237 Profiles of Drug Substances, Excipients, and Related MethodologyKevin Beattie, ... Jasmina Novakovic, in Profiles of Drug Substances, Excipients and Related Methodology, 2013 5 EliminationThe elimination half-life for orally administered Carvedilol is typically 6–7 h [71], although different values are reported by various resources. A study on stereoselective disposition of carvedilol in man, conducted by Neugebauer et al. [85], revealed differences upon oral administration of 50 mg dose in the elimination half-lives between two enantiomers with values of 9.6 and 22.1 h for R(+) and S(−) enantiomers, respectively. Carvedilol is eliminated mainly by hepatic metabolism and most of the metabolites are excreted into the bile and eliminated via feces. Only about 16% of carvedilol or its metabolites are excreted in the urine [72,81]. In summary, carvedilol is a racemic drug, with S(−) and R(+) enantiomers differing significantly in their pharmacodynamic activities and PK properties. Enantioselective bioanalytical methods are needed to study disposition and metabolism of individual enantiomers. What does drug halfThe half-life of a drug is the time it takes for the amount of a drug's active substance in your body to reduce by half. This depends on how the body processes and gets rid of the drug. It can vary from a few hours to a few days, or sometimes weeks.
What factors can extend the halfThe two major factors that affect drug half-life include:. Kinetics: Two types of elimination kinetics may affect the half-life: First-order kinetics: The clearance rate directly depends on the initial concentration. ... . Age: The half-life of drugs increases with age.. How does halfA dosing interval of about a half-life is appropriate for drugs with half-lives of approximately 8-24 hours allowing dosing once, twice or three times daily. It is usually not practicable to administer drugs with shorter half-lives more frequently.
What is a drug halfDrug half-life is defined as the amount of time required for 50% of a drug to: be eliminated by the body.
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