How semaglutide works: the GLP-1 receptor

Semaglutide is a GLP-1 receptor agonist (GLP-1 stands for glucagon-like peptide-1, a hormone your gut produces after you eat). The drug is sold as Ozempic and Rybelsus for type-2 diabetes and as Wegovy for chronic weight management. This article explains how semaglutide produces its effects at the receptor level, what happens downstream of that receptor, and why those effects translate into lower blood glucose and lower body weight.

This is an educational overview for readers who want to understand the pharmacology. It is not medical advice. Any decision about starting, adjusting, or stopping semaglutide belongs with a clinician who knows your history.

The receptor: GLP-1R

GLP-1 is an incretin hormone, meaning it is released by cells in the small intestine in response to food and signals to other organs that a meal is underway. Its main receptor, GLP-1R, sits on the surface of several cell types: beta cells in the pancreas, cells in the brain and brainstem, cells in the stomach wall, and cells lining blood vessels. Natural GLP-1 is broken down within minutes by an enzyme called DPP-4; semaglutide was engineered to resist that breakdown^[1]. The result is a molecule with a half-life of about one week, which is why once-weekly injection is pharmacologically appropriate. Tirzepatide is a related drug that adds a second receptor target (GIP) on top of GLP-1R, which is why the two are often compared.

Pancreatic effects: glucose-dependent insulin release

When semaglutide binds GLP-1R on pancreatic beta cells, it triggers insulin secretion, but only when blood glucose is elevated. This is the glucose-dependence property of the GLP-1 class, and it matters because it means the insulin release switches off when glucose returns to normal, which limits the risk of hypoglycaemia (dangerously low blood sugar) on monotherapy^[1]. At the same time, semaglutide suppresses glucagon release from alpha cells (glucagon is the hormone that tells the liver to release stored glucose), so the liver produces less glucose between meals.

Brain effects: appetite and food intake

GLP-1R is expressed in the hypothalamus and brainstem, the regions that regulate hunger and satiety. When semaglutide acts on these receptors it increases feelings of fullness and reduces appetite and food cravings, as the EMA describes in the Wegovy authorisation^[2]. Gastric emptying (the rate at which food leaves the stomach) also slows, which extends the sensation of fullness after a meal. The combined effect is that people on semaglutide tend to eat less at each sitting and feel less driven to eat between meals.

These central effects on appetite are what produced the weight-loss results in the STEP-1 trial: Wilding et al. (NEJM, 2021) reported a mean body-weight reduction of 14.9% at 68 weeks in participants on semaglutide 2.4 mg plus lifestyle intervention, compared with 2.4% on placebo plus the same lifestyle support^[3]. At the time, this was the largest mean weight loss seen in any non-surgical obesity trial.

Cardiovascular effects

GLP-1R is also expressed on the heart and blood-vessel cells. The SELECT trial result is covered in detail in the semaglutide evidence section. In brief: the SUSTAIN-6 trial (Marso et al., NEJM, 2016) found a statistically significant reduction in major adverse cardiovascular events in people with type-2 diabetes at elevated cardiovascular risk: 6.6% of participants in the semaglutide arm versus 8.9% in the placebo arm over 104 weeks^[4]. The SELECT trial (Lincoff et al., NEJM, 2023) then extended this finding to people with obesity but without diabetes: among 17,604 adults, semaglutide 2.4 mg reduced the rate of cardiovascular death, non-fatal heart attack, or non-fatal stroke to 6.5% versus 8.0% on placebo, a hazard ratio of 0.80^[5]. The mechanism behind the cardiovascular benefit is not fully characterised, but proposed pathways include direct effects on cardiac and vascular GLP-1R, and indirect effects from weight and glucose reduction.

What the drug does not do

Semaglutide does not cure the underlying conditions it is used for. Weight tends to return when people stop the drug, as demonstrated by the STEP-4 discontinuation trial: participants who had lost weight over 20 weeks and then switched to placebo regained most of it. The retinopathy signal seen in SUSTAIN-6 (a higher rate of diabetic eye complications in the semaglutide group) is attributed partly to rapid HbA1c reduction and is worth monitoring in people with pre-existing retinopathy. Gastrointestinal side effects, primarily nausea, vomiting, and diarrhoea, are dose-dependent and are the most common reason for discontinuation in trials. For a full picture of the safety data and what is still unknown, see the semaglutide safety overview.

The analogy: it mimics a meal signal

The clearest plain-English frame for semaglutide is that it mimics the signal your gut sends after eating, held at a sustained level throughout the week rather than the few minutes natural GLP-1 lasts. The pancreas reads that signal and releases insulin proportional to blood glucose. The brain reads that signal and dials down hunger. The stomach reads that signal and slows its emptying. The net effect is improved glycaemic control and, at the higher dose, meaningful weight reduction. For details on regulatory status by country, see the semaglutide peptide page. For a comparison with the dual-agonist class, see the article on Mounjaro and Zepbound.