§ Glossary · pharmacology

Half-life

The time required for plasma concentration of a compound to fall to half its initial value.

Half-life (t½) is the time required for the plasma concentration of a compound to fall to half its initial value. It's a fundamental pharmacokinetic parameter that determines dosing frequency: a drug typically reaches steady state after 4-5 half-lives, and steady-state concentrations are predictable from dose × frequency × half-life. Natural peptides have very short half-lives (GLP-1: ~1-2 min; GHRH: ~7 min) due to rapid enzymatic degradation. Engineered research peptides extend half-life through modifications: lipidation (semaglutide ~7 days), albumin binding (CJC-1295 with DAC ~6-8 days), Fc fusion, or pegylation.

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