Semaglutide info: what the medicine actually does and who it suits

How does semaglutide reduce body weight?

Semaglutide binds to and activates the GLP-1 receptor, a hormone pathway that plays several roles at once. It signals the brain that you have eaten, reducing appetite and the urge to eat beyond what you need. It slows how quickly food passes from the stomach into the small intestine (a process called gastric emptying) which means meals feel satisfying for longer. It also influences blood-sugar regulation by promoting insulin release in response to food and reducing glucagon.

The practical result for most people is a significant and sustained reduction in calorie intake, without the conscious effort of strict dieting. This is not a stimulant or a fat-burning compound; it works with your own hormonal signalling rather than overriding it. You can read a fuller clinical overview on the Wegovy treatment page, which covers both the injection and the newer tablet form approved by the MHRA in June 2026.

A question our prescribers hear most weeks is whether semaglutide simply suppresses appetite the way older diet pills did. It does not. GLP-1 receptor activation affects satiety signalling at a neurological level, not through stimulant effects, the mechanism is meaningfully different, which is one reason the clinical trial results are considerably stronger than those seen with older appetite suppressants.

What does the trial evidence say about how much weight people lose?

The headline figure from the STEP 1 trial, published in the New England Journal of Medicine, is an average weight reduction of around 15% over 68 weeks at the 2.4 mg weekly injection dose, which is the maintenance level reached after a structured upward titration, compared with around 2.4% on placebo, both groups alongside lifestyle support. That is a meaningful difference by any clinical standard.

The oral form adds another layer of evidence. The OASIS 4 trial, covering 64 weeks in adults without diabetes, found an average weight loss of approximately 13.6% on the 25 mg daily tablet, the highest approved oral dose, against roughly 2.4% on placebo. Among participants who adhered fully throughout, the figure was closer to 17%. That distinction matters: the 17% reflects ideal-adherence conditions, not the average real-world picture, so it is the 13.6% that gives a more honest baseline.

Semaglutide also produced a greater average weight reduction than placebo in participants with type 2 diabetes and in those with cardiovascular disease in separate trials. Results vary between individuals (biological responses differ, as do starting points) so no outcome can be guaranteed. What the data does confirm is that the effect is reproducible at population scale and clinically significant.

For a broader look at how these figures compare across licensed weight-loss treatments, the weight-loss treatment overview sets them in context.

Who meets the eligibility criteria for semaglutide in the UK?

The licensed criteria for semaglutide (Wegovy injection) cover adults with a BMI of 30 or above, or a BMI of 27 to 29.9 alongside at least one weight-related health condition, for example, high blood pressure, raised cholesterol, prediabetes, type 2 diabetes or obstructive sleep apnoea. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance, reflecting differences in cardiometabolic risk at a given BMI.

The NHS route is more restricted. NICE's recommendation for Wegovy (TA875) limits NHS use to adults within a specialist weight management service, for a maximum of two years, with a BMI of at least 35 (or lower for some ethnic backgrounds) and at least one weight-related comorbidity. Waiting lists for NHS specialist services are long in most areas.

Private prescription removes the NHS eligibility thresholds but not the clinical ones. A prescriber still reviews the full picture: current medications, medical history, contraindications and whether the medicine is appropriate. BMI alone is never the deciding factor. Those who are pregnant, breastfeeding, trying to conceive or under 18 are not suitable candidates. People with a personal or family history of medullary thyroid carcinoma or MEN2, or with active pancreatitis, also require careful prescriber discussion before any GLP-1 medicine is considered.

Our FAQs page covers some of the most common eligibility questions in plain terms.

What side effects should people taking semaglutide be aware of?

The most commonly reported effects are gastrointestinal: nausea, vomiting, diarrhoea, constipation, reflux and burping. These tend to be most noticeable after starting or following a dose increase, and they settle for most people within one to two weeks as the body adjusts. The NHS medicines page for semaglutide lists the full side-effect profile with guidance on what to watch for.

More serious but uncommon effects include acute pancreatitis. Seek urgent medical attention for severe or persistent stomach pain (particularly pain that radiates into the back) with or without vomiting. Gallbladder problems, injection-site reactions and, in people with pre-existing diabetic eye disease, changes in retinopathy are also documented. Dehydration from prolonged GI illness can occasionally affect kidney function, so staying adequately hydrated matters.

Semaglutide is not recommended during pregnancy, while breastfeeding, or when actively trying to conceive. Women using oral contraceptives starting tirzepatide (a related medicine) are advised to add a barrier method temporarily, though current evidence has not shown the same pill-absorption interaction with semaglutide 1 mg, the dose used in type 2 diabetes treatment, or with higher weight-management doses specifically. Discuss contraception and HRT with your prescriber before starting any GLP-1 medicine. You can report any suspected side effects or concerns about your treatment directly through the MHRA Yellow Card scheme.

If you have questions about how cost and access fit together, the Wegovy price comparison page sets out what legitimate private treatment typically involves and what a transparent price should include.

Semaglutide is a prescription-only medicine. If you think it could be right for you, the right next step is a clinical assessment. Check your eligibility with our prescribers, the consultation is free, and a GPhC-registered clinician reviews every application the same day.