Semaglutide is the molecule that pulled GLP-1 receptor agonism from a niche diabetes therapy into one of the most-prescribed drug classes in modern medicine. Of the three commonly-discussed GLP-1 peptides — Semaglutide, Tirzepatide, Retatrutide — Semaglutide has the deepest human dataset, the longest post-marketing surveillance window, and the most stable dose-response characterization. If you're trying to understand the GLP-1 class, this is the right molecule to start with.
Mechanism
Semaglutide is a synthetic 31-amino-acid analogue of glucagon-like peptide-1, modified at two positions and conjugated to a fatty-acid side chain. The modifications matter for two reasons. First, position-8 substitution (alanine to alpha-aminoisobutyric acid) protects the molecule from DPP-4 cleavage, the enzyme that destroys native GLP-1 within minutes. Second, the C-18 fatty diacid linker drives reversible binding to serum albumin, which extends circulating half-life from minutes to roughly a week.
Once in circulation, Semaglutide binds the GLP-1 receptor on pancreatic beta cells (driving glucose-dependent insulin secretion), on the central nervous system (suppressing appetite via hypothalamic and brainstem pathways), and on gastric tissue (slowing gastric emptying). The appetite-suppressive mechanism — not the insulinotropic one — accounts for most of the body composition response in non-diabetic users.
Pharmacokinetics
Subcutaneous Semaglutide reaches peak plasma concentration at 1 to 3 days post-dose, with a terminal half-life of approximately 165 hours (around 7 days). This is the basis of the once-weekly dosing schedule. Steady-state concentrations are reached after 4 to 5 weeks of continuous dosing — a practical implication being that the user who quits at week 3 because "it's not working" hasn't yet tested the compound at therapeutic plasma levels.
Bioavailability after subcutaneous injection is approximately 89%. Site of injection (abdomen, thigh, upper arm) does not significantly affect absorption in the published comparisons.
Dose-response
The clinical literature has characterized the dose-response curve well. Starting doses of 0.25 mg weekly are titrated upward at four-week intervals to a maintenance range of 1.0 to 2.4 mg weekly, with the higher end driving the larger composition responses observed in the . Above approximately 2.4 mg weekly, the curve flattens — incremental dose increases produce diminishing returns and increased GI side-effect burden.
