What is half-life and why does it matter?

Half-life (t½) is the time it takes for the plasma concentration of a compound to fall to half of its starting value. It is a compact way to describe the pharmacokinetic behaviour of a compound. From half-life you can predict several practical things: how long a single dose keeps producing effect (roughly 4–5 half-lives), how often to dose for a steady blood level (dosing intervals of about one half-life produce roughly two-fold peak-to-trough variation), and how long a compound persists after the last dose (roughly 4–5 half-lives to clear substantially).

Native peptide hormones typically have very short half-lives — native GLP-1 is about 2 minutes because DPP-4 rapidly degrades it. This is pharmacologically inconvenient because it would require continuous infusion. The engineered peptide drugs in this catalog have much longer half-lives thanks to specific chemistry: semaglutide's fatty-acid conjugation binds it to serum albumin, extending half-life to about 165 hours (a week), which is what allows weekly dosing. CJC-1295 DAC uses covalent albumin binding to extend half-life similarly. CJC-1295 No DAC has hours of half-life, giving pulsatile pharmacology that preserves GHRH-receptor responsiveness.

Half-life is not always the whole story. Some peptides continue producing effects after plasma levels have fallen — SS-31 concentrates 1000-fold in mitochondria and its effect duration is governed by mitochondrial clearance. Some peptides trigger cascades that persist beyond their own clearance — kisspeptin's brief pulse triggers a GnRH-LH cascade that plays out over hours. Read half-life alongside mechanism for the fullest picture.