Peptides - Compound guides
Semaglutide
Semaglutide is a GLP-1 receptor agonist and a prescription-only medicine in the UK, available only on prescription after clinical assessment. This page explains the pharmacology and the trial record; it is not a recommendation to obtain or use it.
Semaglutide is a synthetic analogue of the human gut hormone GLP-1. The natural hormone is destroyed within a couple of minutes by an enzyme called DPP-4, so semaglutide's sequence was altered at the site the enzyme attacks and a fatty acid chain was attached that allows the molecule to bind to albumin in the blood. Together these changes extend its working life from minutes to about a week. It was developed for type 2 diabetes and later studied and licensed for weight management. In the UK it is marketed under brand names including Ozempic, Wegovy and Rybelsus, named here only for recognition.
Semaglutide is a licensed prescription-only medicine in the UK, and that legal status is set out in full below.
How it works
GLP-1 is one of the incretin hormones, released by L-cells in the lining of the small intestine when food arrives. Its purpose is to coordinate the body's response to a meal. It stimulates the pancreas to release insulin in a glucose-dependent manner - meaning the effect scales with how high blood glucose actually is, which limits the tendency to drive glucose too low on its own. It suppresses glucagon, the hormone that instructs the liver to release stored glucose. It slows gastric emptying, so a meal leaves the stomach more gradually and the rise in blood glucose after eating is blunted. And it acts on GLP-1 receptors in the hypothalamus and brainstem, regions that regulate hunger and satiety, reducing appetite and the drive to eat.
Semaglutide is a receptor agonist: it binds the GLP-1 receptor and activates it in the same way the natural hormone does, but continuously rather than in brief post-meal pulses. That sustained activation is the source of both the therapeutic effects and the side effects - the same slowing of gastric emptying that blunts glucose spikes is also why nausea is the most common complaint. Unlike tirzepatide it does not act at the GIP receptor; it is a single-receptor drug.
What the research actually shows
The SUSTAIN programme tested semaglutide in adults with type 2 diabetes, with change in HbA1c as the primary endpoint, comparing it against placebo and against active comparators including other glucose-lowering drugs. Across the SUSTAIN trials semaglutide produced changes in HbA1c against placebo and against active comparators, with weight reduction reported as a secondary outcome. What those results mean for any individual is a matter for a prescriber.
The STEP programme tested semaglutide for weight management. In the phase 3 STEP 1 trial, adults with obesity or with overweight plus at least one weight-related condition, and without diabetes, were randomised to placebo or semaglutide alongside lifestyle intervention and treated for 68 weeks, with percentage change in body weight as the primary endpoint. Participants receiving semaglutide lost substantially more weight than those receiving placebo. Other trials in the programme studied adults with type 2 diabetes, where the average weight effect was smaller, and a withdrawal trial in which participants who stopped treatment regained much of the weight they had lost over the following year.
The SELECT trial is a different kind of study. It was a cardiovascular outcomes trial in adults with established cardiovascular disease and overweight or obesity, but without diabetes, randomised to semaglutide or placebo and followed for several years. Its primary endpoint was not weight but a composite of cardiovascular death, non-fatal heart attack and non-fatal stroke, and the trial reported a significant reduction in that composite endpoint in the semaglutide group compared with placebo. That is a hard clinical endpoint rather than a measure of weight alone. It applies to the population studied - people with existing cardiovascular disease - and should not be read across to everyone.
Risks, side effects and what is still unknown
- Gastrointestinal effects are the most common adverse events across the trial programmes - nausea, vomiting, diarrhoea, constipation and abdominal pain - and were the usual reason for discontinuing treatment.
- Pancreatitis, gallbladder disease including gallstones, and thyroid C-cell tumours in rodent studies are the labelled concerns for the GLP-1 class. Product information carries a contraindication in people with a personal or family history of medullary thyroid carcinoma or MEN2, and the human relevance of the rodent finding is still not settled.
- Diabetic retinopathy complications were reported more often in the semaglutide group in one of the SUSTAIN trials, thought to relate to rapid improvement in glucose control, and eye assessment is part of appropriate prescribing in people with diabetes.
- Hypoglycaemia risk increases when combined with insulin or sulfonylureas, which is one reason a prescriber must review all other medicines.
- Loss of lean mass accompanies fat loss. Body composition studies show a meaningful fraction of the weight lost is not fat, with implications for muscle mass and function.
- Weight regain after stopping is documented directly in the STEP withdrawal trial. The effect is dependent on continued treatment.
- Effects in pregnancy are not established, and very long-term outcomes over decades remain unknown even for the best-studied drug in this class.
Legal status in the UK
Semaglutide is a licensed prescription-only medicine in the UK. It is legally available only via a prescription issued after a clinical assessment by an appropriately qualified prescriber, and dispensed by a registered pharmacy. There is no lawful route to it that bypasses that assessment. Supplying a prescription-only medicine without a prescription is a criminal offence, and a website offering it without one is operating outside the law whatever its stated jurisdiction.
The MHRA has issued repeated public warnings about counterfeit and falsified GLP-1 pens circulating outside the regulated supply chain, sold through social media, private sellers and non-pharmacy outlets. Some falsified pens seized in the UK were found to contain insulin instead of semaglutide, and people who injected them were hospitalised, in some cases in a coma from severe hypoglycaemia. Others have been found non-sterile, wrongly dosed or containing no active drug. Purity, sterility, dose accuracy and cold-chain integrity are exactly what medicines regulation exists to guarantee, and none of it applies to a product bought outside a registered pharmacy. Semaglutide is also prohibited in sport under World Anti-Doping Agency rules for athletes subject to testing.
This page is educational information only and is not medical advice. Semaglutide is a prescription-only medicine and nothing on this page should be read as a recommendation to take, obtain or consider it. Decisions about weight, diabetes or cardiovascular risk belong with a qualified healthcare professional who can assess your individual circumstances.
Metabolic and incretin compounds
Start with the overview: How research peptides are grouped into families
Further reading
Sources for this classroom
Educational content only. Not medical advice, diagnosis or treatment. Always consult a qualified healthcare professional before changing your health regimen.
