Definition
The half-life of a drug 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: t 1/2 = 0.693·V ss /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 given. 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 provide an indication of the expected time to the onset of withdrawal/discontinuation symptoms to occur if the drug is associated with withdrawal/discontinuation symptoms.
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Rey, J.A. (2011). Half-Life. In: Kreutzer, J.S., DeLuca, J., Caplan, B. (eds) Encyclopedia of Clinical Neuropsychology. Springer, New York, NY. https://doi.org/10.1007/978-0-387-79948-3_1739
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DOI: https://doi.org/10.1007/978-0-387-79948-3_1739
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