Semaglutide: what it is, what it does, and what the evidence says

What the STEP 1 trial actually showed

The clearest starting point for understanding semaglutide is the data that changed clinical opinion. The STEP 1 trial, published in the New England Journal of Medicine, enrolled nearly 2,000 adults with obesity and no diabetes, giving half weekly 2.4mg semaglutide injections alongside lifestyle support. After 68 weeks, participants on semaglutide lost an average of around 15% of their body weight; those on placebo lost roughly 2.4%. That gap (about 12 percentage points) was large enough to prompt regulators, then NICE, to take formal action.

What makes the result clinically meaningful rather than statistically interesting is the mechanism behind it. Semaglutide binds to GLP-1 receptors in the hypothalamus, reducing the hunger signalling that drives overconsumption. It also slows how quickly the stomach empties, so a smaller meal produces a fuller feeling for longer. This isn't willpower substitution. It's a pharmacological shift in the biological signals that govern appetite, which is why the weight loss seen in trials was substantially greater than lifestyle modification alone.

Semaglutide at 2.4mg is the active ingredient in Wegovy. If you want a fuller picture of what Wegovy is and how it's used, that page covers the pen formats, titration schedule and MHRA approval in more detail. The evidence base is the place to start; if you'd like to read more about semaglutide and how it works, that page is a useful companion before moving on to the practical questions.

What semaglutide is used for in the UK, the licensed indications

Semaglutide holds more than one UK licence, and the distinction matters. As Wegovy (2.4mg weekly injection), it is licensed for weight management in adults with a BMI of 30 or above, or 27 to 29.9 with at least one weight-related condition such as high blood pressure, high cholesterol or prediabetes. A higher 7.2mg weekly dose was approved by the MHRA in January 2026, initially via three 2.4mg pens, with a dedicated single-dose pen approved in April 2026. A daily oral tablet (the first oral GLP-1 medicine licensed for weight management in the UK) was approved in June 2026 under the same Wegovy brand. For a full breakdown of what semaglutide is licensed for, including the cardiovascular and liver-disease indications approved more recently, that page covers the detail.

Ozempic, also semaglutide but at lower doses (0.5mg and 1mg weekly), is licensed for type 2 diabetes in the UK. It is not licensed for weight loss. Rybelsus tablets (3mg, 7mg, 14mg) are oral semaglutide for type 2 diabetes, not the same as Wegovy tablets. These distinctions are regularly misunderstood online, and if you are wondering what semaglutide medicines are and how they differ from one another, that page sets out the key differences clearly. The practical consequence: anyone using Ozempic or Rybelsus specifically for weight management is doing so off-label, without the supporting trial infrastructure or regulatory oversight that the weight-loss licence provides.

How semaglutide compares within the GLP-1 class

Semaglutide is a GLP-1 receptor agonist, it works on a single gut-hormone pathway. Tirzepatide (Mounjaro), the other medicine in this class widely used for weight management in the UK, activates two receptors: GLP-1 and GIP. In the SURMOUNT-5 head-to-head trial published in the New England Journal of Medicine in 2025, tirzepatide produced greater average weight reduction than semaglutide 2.4mg over 72 weeks. The newer 7.2mg semaglutide dose narrows that gap; the two medicines are increasingly close in performance at their highest approved strengths.

That said, head-to-head data informs a decision; it doesn't make one. Which medicine suits a particular person depends on their medical history, any existing conditions, how they respond to the starting dose, and practical factors like whether they prefer a daily tablet or a weekly injection. If you're weighing up the options and want to understand the full range of licensed weight-loss treatments, that's worth reading before a consultation. And if cost is part of the picture, a realistic overview of what Wegovy costs in the UK is a useful companion to the clinical information.

One other thing worth knowing: semaglutide is a Black Triangle medicine (▼), meaning the MHRA collects additional safety data on it. That's standard for newer medicines and doesn't mean it's considered unsafe, it means the evidence base is still being built. Patients can report any suspected side effects at the MHRA's Yellow Card scheme.

What a prescriber looks at before semaglutide is appropriate

A lot of people arrive at this question already knowing they want to try semaglutide. That's understandable. The evidence is striking, the media coverage has been relentless, and for many people the frustration with waiting lists or with treatments that haven't worked is entirely real. But semaglutide being well-evidenced doesn't mean it's right for every individual, and the prescriber's job is to assess that properly.

The clinical checklist covers current medical conditions (including any personal or family history of medullary thyroid carcinoma or MEN2 syndrome, where semaglutide is contraindicated), other medicines being taken, and whether the person has a history of pancreatitis or certain gastrointestinal conditions. Pregnancy, breastfeeding and trying to conceive are all circumstances in which semaglutide isn't recommended. The treatment is not licensed for under-18s. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance, so the 30 and 27 figures aren't the only reference points. It is also worth understanding what wegovy withdrawal symptoms can occur if treatment is stopped, so that anyone considering the medicine can plan for the longer term from the outset.

At nume, every consultation is read the same day by a GPhC-registered Independent Prescriber, a real clinician who works through your medical history rather than an automated system. You can read more about our clinical team and the standards we hold ourselves to. If semaglutide is clinically suitable and you'd like to explore starting treatment, speaking to our prescribers is the straightforward next step.