Half-life

Half-life (t½) is the time required for the plasma concentration of a compound to fall to half its previous value. For peptide research, half-life governs three practical decisions: dosing frequency, time to steady state, and the overlap window when stacking compounds.

After approximately five half-lives, a compound is considered effectively eliminated (≈97%). Conversely, a compound dosed at fixed intervals reaches steady state after about 4–5 half-lives. That is why semaglutide (t½ ~7 days) takes roughly 5 weeks to reach steady state, while BPC-157 (t½ ~4 hours) is at steady state within a single day.

Plasma half-life is not always the same as effect half-life. DSIP and Semax both have very short systemic half-lives but produce CNS effects that outlast their measurable plasma concentration; TB-500’s effects on tissue regeneration similarly outlast plasma levels.