How long it takes for half the compound to clear from your blood — a measure of how long it stays active.Most endogenous peptides have half-lives of minutes due to proteolytic degradation. Medicinal chemistry modifications extend half-life: D-amino acid substitutions block proteolysis, PEGylation reduces renal clearance, albumin binding via DAC leverages albumin’s 19-day half-life. Important: plasma half-life is not the same as duration of biological effect. A compound cleared from the blood may have already activated signaling cascades that persist for hours or days. Conversely, a long plasma half-life does not guarantee prolonged receptor activation.