Encyclopedia of Clinical Neuropsychology

2018 Edition
| Editors: Jeffrey S. Kreutzer, John DeLuca, Bruce Caplan


  • Jose A. ReyEmail author
Reference work entry
DOI: https://doi.org/10.1007/978-3-319-57111-9_1739


The half-life of a drug (t1/2) is the time it takes for the blood or plasma concentration of a given drug to be reduced by 50%.

The formula for calculating this value is t1/2 = 0.693 · Vss/CL.

This pharmacokinetic value is also important for determining the time to steady-state concentrations in the body. It takes approximately five half-lives for a drug to achieve steady-state plasma concentrations in the body, which signifies that the amount of the drug being eliminated from the body is equal to the amount of the drug being administered. Subsequently, a medication usually requires approximately five half-lives for a significant majority (~97%) of the drug to be eliminated from the body after discontinuation. The half-life of a drug may influence or dictate the daily dosage regimen of a particular agent and also the amount of time needed to achieve a needed and consistent response for efficacy; and a medication’s (or drug of abuse) half-life may also provide an indication...

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References and Readings

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Copyright information

© Springer International Publishing AG, part of Springer Nature 2018

Authors and Affiliations

  1. 1.College of PharmacyNova Southeastern UniversityFt. LauderdaleUSA