Half-life (t½) is the time required for a concentration to fall to half its starting value. Under first-order elimination the fraction removed per unit time is constant, so a compound is roughly 97% cleared after five half-lives regardless of the starting amount. Reported values are usually terminal half-lives, taken from the final log-linear slope of a concentration curve, and they are properties of a species and route rather than of the molecule alone.
Why half-life matters in peptide research
Peptides span an unusually wide range. Native GLP-1 is degraded by DPP-4 within about two minutes; the approved drug semaglutide, engineered with a fatty-acid chain for albumin binding, has a human half-life reported near one week. That is a roughly five-thousand-fold difference produced by structural modification, and it is the reason strategies such as DAC and PEGylation exist at all.
Two confusions are worth avoiding. First, plasma half-life describes clearance from circulation, not stability of a reconstituted vial in a refrigerator, which is governed by hydrolysis and oxidation instead. Second, a long half-life does not imply a long duration of measurable effect, because receptor occupancy and downstream signalling have their own timescales. Background is in the stability and half-life guide.