Semaglutide

Semaglutide mechanism of action

Semaglutide is a GLP-1 receptor agonist: it mimics the gut hormone glucagon-like peptide-1 and activates GLP-1 receptors in the pancreas, brain and gut^[1]. In the pancreas this increases insulin release only when blood glucose is raised and lowers glucagon secretion, which improves blood-glucose control. In the brain, GLP-1 signalling increases satiety and reduces hunger, so people tend to eat less energy overall^[2]. Semaglutide can also slow gastric emptying, most noticeably early in treatment. Together, these effects explain its clinically established benefits on blood glucose and body weight.

1. Semaglutide binds to and activates GLP-1 receptors.
2. Pancreatic insulin secretion increases in a glucose-dependent manner.
3. Glucagon secretion is reduced when glucose is elevated.
4. Central appetite and satiety pathways reduce hunger and energy intake.
5. Gastric emptying can be slowed, particularly early in treatment.
6. Together, these effects improve glucose control and can support weight loss.

Mechanism explains how a drug acts; it is not by itself evidence of benefits beyond those shown in clinical trials.

How does semaglutide cause weight loss?

Weight loss with semaglutide comes mainly from eating less. By activating GLP-1 receptors in brain regions that regulate appetite, it reduces hunger, increases feelings of fullness and lowers food cravings, so total energy intake falls. In a controlled study in adults with obesity, semaglutide 2.4 mg reduced ad-libitum energy intake substantially compared with placebo, alongside lower hunger and better reported control of eating^[2]. Weight loss is therefore not simply the result of "slowing the stomach": gastric emptying is one contributor, but reduced appetite and energy intake driven by central GLP-1 signalling are the dominant factors. Effects depend on continued treatment, and weight regain is common after stopping.

Does semaglutide work by slowing gastric emptying?

Partly, but not mainly. Semaglutide can delay gastric emptying, and this effect is most apparent soon after starting treatment^[1]. In the study above, gastric emptying measured over the first hour was delayed, while overall emptying across five hours was not meaningfully changed^[2]. Its longer-term weight and metabolic effects are better explained by central appetite regulation, increased satiety, reduced energy intake and pancreatic glucose regulation.

How does semaglutide affect insulin and glucagon?

Semaglutide increases insulin secretion in a glucose-dependent way: it amplifies insulin release when blood glucose is high, and this effect fades as glucose returns towards normal^[1]. It also lowers glucagon secretion when glucose is elevated, reducing the liver's glucose output. This differs from treatments that force insulin release regardless of glucose level, which is why semaglutide on its own carries a relatively low risk of hypoglycaemia; the risk rises when combined with insulin or sulfonylureas^[1].

What is semaglutide structurally?

Semaglutide is a modified analogue of human GLP-1. Native GLP-1 is broken down by the enzyme DPP-4 and cleared within minutes. Semaglutide has an amino-acid substitution that protects it from DPP-4 and a fatty-acid side chain that binds to albumin in the blood, slowing clearance. Together these changes give it a half-life of about a week^[3][4] — see why peptide half-life matters.

Structure: GLP-1 analogue, 94% homology to human GLP-1

Quick answers

Does semaglutide increase insulin?

Yes, but only in a glucose-dependent way — mainly when blood glucose is elevated.

Does semaglutide reduce glucagon?

Yes, it lowers glucagon secretion when glucose is high, which helps control blood glucose.

Does semaglutide slow gastric emptying?

It can, mostly early in treatment; it is a minor contributor compared with reduced appetite.

For reconstitution maths only (no dose recommendations), see the Semaglutide calculator.

Mechanism sources

1. [1] FDA prescribing information — Wegovy (semaglutide) injection, 2021 FDA prescribing information
2. [2] Friedrichsen et al., Diabetes Obes Metab 2021 — semaglutide 2.4 mg: energy intake, appetite and gastric emptying in adults with obesity Randomised human trial
3. [3] Lau et al., J Med Chem 2015 — discovery of the once-weekly GLP-1 analogue semaglutide Mechanistic study
4. [4] Knudsen & Lau, Front Endocrinol 2019 — the discovery and development of liraglutide and semaglutide Narrative review

Reported benefits

Practical considerations

Side effects and risks

Semaglutide dosing overview

Semaglutide FAQ

Research references