Half-life is the oldest pharmacokinetic parameter and all clinicians believe they understand its relevance. The t 1/2, op is compared with previously proposed half-lives for predicting accumulation. 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. 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. 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. 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 1/2s for eight orally dosed drugs with t 1/2,z values longer than one day. However, accumulation at steady state may be markedly over predicted utilizing t 1/2, z. Alternatively, terminal half-life ( t 1/2,z) is utilized as the single defining t 1/2 for most drugs. 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.
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