The half-life of a drug is the time required for its plasma concentration to decrease by 50%. This parameter is crucial because it reflects the rate of drug elimination from the bloodstream, impacting dosing intervals and steady-state levels. It does not indicate the time to reach maximum effect or absorption but specifically measures the decline in plasma levels. Understanding half-life helps clinicians optimize therapy by predicting how long a drug stays active in the body and when the next dose should be administered to maintain therapeutic levels without toxicity.

Reference: Goodman & Gilman’s The Pharmacological Basis of Therapeutics, Brunton et al., 13th Edition.