Semaglutide is a GLP-1 receptor agonist: a 94%-homologous copy of the human hormone GLP-1 that binds and activates the same receptor. That part is settled, and it is the whole of what the label asserts without qualification about the mechanism.
Everything downstream of it is stated far more carefully than you would guess from reading about the drug. The sentence that explains weight loss contains the word likely. For three of its approved indications the label says the mechanism has not been established. And one listed pharmacodynamic effect could not be reproduced in the trial that looked for it at 20 weeks.
What the label claims, and how firmly
Quotations are from sections 12.1 and 12.2 of the Wegovy and Ozempic prescribing information. “Hedged” means the label itself qualifies the claim — not that the effect is disputed.
The glucose mechanism is the well-evidenced one
Ironically, the part of semaglutide almost nobody searches for is the part the label backs with numbers. It lowers glucose by stimulating insulin secretion and lowering glucagon secretion, both glucose-dependently — when blood glucose is high, insulin is stimulated and glucagon is inhibited; when it is not, neither happens much.
After 12 weeks at 1 mg in patients with type 2 diabetes, the Ozempic label reports reductions against placebo of 29 mg/dL (22%) in fasting glucose, 74 mg/dL (36%) in 2-hour postprandial glucose and 30 mg/dL (22%) in mean 24-hour glucose. Glucagon fell 8% fasting, 14–15% in the postprandial response and 12% across 24 hours.
Glucose-dependence is also why semaglutide on its own is not a hypoglycaemia drug: during induced hypoglycaemia it did not alter the counter-regulatory rise in glucagon compared with placebo, and did not impair the fall in C-peptide. The risk appears when it is combined with insulin or a sulfonylurea, neither of which waits to be asked.
The weight sentence, and the word doing the work
Section 12.2 of the Wegovy label gives weight loss three sentences in total: Semaglutide lowers body weight with greater fat mass loss than lean mass loss. Semaglutide decreases calorie intake. The effects are likely mediated by affecting appetite.
Read that as a regulator would. The first two are flat statements of measured fact. The third is a hypothesis about why, and the FDA does not write “likely” about something that has been demonstrated. The mechanism section supports it indirectly: GLP-1 is described as a physiological regulator of appetite and caloric intake, GLP-1 receptors are present in several areas of the brain involved in appetite regulation, and animal studies show semaglutide distributing to and activating neurons in brain regions involved in the regulation of food intake. Animal studies is the label’s own wording.
So the honest version is: it reduces how much you eat, this is measured and large, and the appetite pathway is the best-supported explanation rather than a proven one. The two trials that measured appetite directly, and the weeks they measured it.
Gastric emptying: on the label, absent at week 20
“It slows your stomach” is the explanation most people are given, and it has a genuine basis: the Wegovy label lists delayed gastric emptying as a pharmacodynamic effect, flatly. The Ozempic label is already narrower, describing the contribution to glucose lowering as a minor delay in gastric emptying in the early postprandial phase.
Then there is the trial. In a 20-week study of semaglutide 2.4 mg in 72 adults with obesity, gastric emptying was assessed by paracetamol absorption at week 20 and showed no difference from placebo on any endpoint — area under the curve over the first hour p = 0.85, peak concentration p = 0.33, time to peak p = 0.75.
In the same patients, at the same visit, the effect on eating was unambiguous: ad libitum energy intake at lunch was 35% lower than placebo, 1736 kJ against 2676 kJ, and body weight had fallen 9.9% against 0.4%.
Three indications, three admissions
Semaglutide is approved for more than glucose and weight, and for each of the others the label says the mechanism is unknown. Stated plainly, in one place:
- Cardiovascular risk reduction. “The exact mechanism of semaglutide in CV risk reduction in adults has not been established.”
- Kidney-related risk reduction. “The mechanism of kidney-related risk reduction has not been established.”
- MASH. The precise mechanism is “not fully understood and may involve multiple pathways mediated by weight loss and other factors”. The supporting evidence the label offers is a mouse model, and it adds that the relationship between the pathophysiology of MASH in animal models and humans has not been fully established.
None of that makes the outcomes less real — they were measured in outcome trials, which is how approval was granted. It means the drug demonstrably does things nobody can yet explain, and any page that explains them confidently is going beyond its source.
Why it lasts a week
Native GLP-1 survives minutes. Semaglutide lasts about a week because of three deliberate modifications, and the label names the dominant one: albumin binding is the main protraction mechanism, achieved by attaching a C18 fatty di-acid to position 26 through a hydrophilic spacer. Bound to albumin, the molecule is protected from degradation and cleared far more slowly. Position 8 is modified to resist DPP-4, the enzyme that destroys native GLP-1; position 34 is changed so that only one fatty di-acid attaches.
The structure in full, and why it makes semaglutide a peptide rather than a protein. For what the same chemistry means going the other way, half-life and clearance across the GLP-1 class.
Related
- How long semaglutide takes to work — the same pharmacology on a clock.
- Ozempic, Wegovy, Rybelsus — one molecule, six brands
- Semaglutide dosage calculator and full titration schedule