Semaglutide

What it is

Semaglutide is a GLP-1 receptor agonist — a molecule engineered to mimic glucagon-like peptide-1, a hormone the gut releases after eating. It is the compound that, more than any other, turned “GLP-1” from an endocrinology term into a household word. Sold as Ozempic and Rybelsus for type 2 diabetes and as Wegovy for weight management, it is one of the most-studied molecules in this entire encyclopedia — and the reference point against which every newer compound here is measured.

How it works

Native GLP-1 is broken down within minutes. Semaglutide’s structural modifications let it resist that breakdown and circulate for about a week, which is why it’s a once-weekly injection (or a daily oral tablet in the Rybelsus form). Mechanistically it does three things at once: it enhances glucose-dependent insulin secretion (so it lowers blood sugar mainly when blood sugar is high, which limits hypoglycemia), it slows gastric emptying (food stays in the stomach longer, so you feel full), and it acts on appetite centers in the brain to reduce hunger and what users often describe as “food noise.”

The evidence — and why it sits at the top of the spectrum

This is the high end of proof. The SUSTAIN program established its glucose effects in diabetes; the STEP program established its weight effects, with STEP 1 showing roughly 15% average body-weight reduction over 68 weeks — a figure that had little precedent for a drug. The SELECT trial then showed a reduction in major cardiovascular events in people with established heart disease and obesity, moving semaglutide beyond “weight loss” into the kind of hard outcomes mortality data actually cares about. That is a fundamentally different tier of evidence than the preclinical research peptides elsewhere on this site.

The honest caveats

Two matter most. First, it’s a treatment, not a course — the STEP 1 extension showed most lost weight returned within a year of stopping, because the underlying appetite biology reverts. Second, the gray-market and compounding reality: surging demand and periodic shortages spawned a large ecosystem of compounded and research-market “semaglutide,” which has been a repeated subject of FDA warnings over dosing errors, mislabeled salt forms, and purity. Approved semaglutide is a prescription medicine that belongs with a clinician; anything outside that channel carries identity, dosing, and sterility risk that the brand-name trials say nothing about.

Where it stands

Semaglutide is the established benchmark — but no longer the highest number on the board. Tirzepatide posted larger weight figures in head-to-head work, and investigational agents like retatrutide have shown even bigger early numbers. Semaglutide’s edge is the depth of its record: years of data, cardiovascular outcomes, and the widest real-world track record of the group. For most readers it remains the most consequential compound in this encyclopedia — and the clearest example of why “approved, with trials” and “research-market peptide” are not the same category of thing.