Serum Half-Life
The time required for the concentration of a compound in the blood to decrease by 50% from its peak level, reflecting the rate at which it is eliminated from systemic circulation.
Serum half-life is a practical determinant of dosing frequency — a peptide with a 30-minute serum half-life is largely cleared within 2–3 hours, while one with a 7-day half-life (like semaglutide) maintains effective levels with once-weekly dosing. Most unmodified research peptides have very short serum half-lives of minutes to hours, which is why modified analogs with extended half-lives are often developed.
For GH secretagogue peptides, the short serum half-life is actually desirable in some protocols because it mimics the natural pulsatile nature of GH secretion — a brief, sharp stimulus followed by rapid clearance. Peptide engineers use techniques like PEGylation, albumin binding, and Fc fusion to extend serum half-life for compounds where prolonged exposure is therapeutically beneficial.