About Semaglutide: the Science, the Evidence and What to Expect

What the STEP 1 trial actually showed, and what it means

The clearest picture of semaglutide's weight-loss effect comes from the STEP 1 trial, published in the New England Journal of Medicine. Researchers followed 1,961 adults with obesity or overweight plus at least one weight-related condition for 68 weeks. Those taking semaglutide 2.4mg weekly lost an average of around 15% of their starting body weight. The placebo group lost closer to 2.4%. Both groups made the same lifestyle changes, which tells you the medicine itself was doing meaningful work.

A 15% average figure covers a range: some participants lost considerably more, others less. Genetics, starting weight, adherence and lifestyle factors all play a role. The trial data is the honest starting point, not a personal forecast. More recently, the MHRA approved a 7.2mg maintenance dose, and early findings at that level suggest average losses approaching what the highest-dose tirzepatide trials recorded, narrowing a gap that once looked wider. Details of the clinical results for Wegovy are covered separately if you want the full picture.

The consistent theme across the trial programme is that semaglutide works best alongside a reduced-calorie diet and increased activity. The medicine changes appetite biology; what you do with that change still matters.

How semaglutide acts on appetite, the mechanism behind the results

GLP-1 (glucagon-like peptide-1) is a hormone your gut releases after eating. It signals to your brain that you've had enough, slows gastric emptying and reduces the urgency to eat again. Semaglutide binds to GLP-1 receptors and activates them in a sustained way, the weekly injection keeps those signals running at a level your body doesn't naturally maintain.

The practical result is that people often find portions they once considered small feel satisfying, and the mental preoccupation with food (sometimes described as constant food noise) quietens. This is biology, not a personality shift. Researchers studying the mechanism describe it as resetting the appetite set point rather than fighting it. If you'd like a full breakdown of everything about Wegovy, including its history and how it compares to other options, that page covers all of it in one place.

One thing worth knowing: semaglutide slows gastric emptying, which affects how the body absorbs other things taken by mouth. That includes oral contraceptives. For women using the pill, NHS guidance recommends discussing contraceptive options with a prescriber before starting, adding a barrier method during dose changes is commonly advised. It's one of several conversations the consultation is designed to surface.

Who semaglutide (Wegovy) is licensed for in the UK

The UK licence covers adults with a BMI of 30 or above, or 27 to 29.9 with at least one weight-related condition, examples include high blood pressure, high cholesterol, prediabetes, type 2 diabetes or obstructive sleep apnoea. Lower BMI thresholds apply for some ethnic backgrounds under UK clinical guidance. The NHS semaglutide medicines page sets out the full licensed indications clearly.

Semaglutide is not licensed for weight management in people under 18, and it is not recommended during pregnancy, breastfeeding or for those trying to conceive. Anyone with a history of medullary thyroid carcinoma or certain pancreatic conditions needs to discuss this with a prescriber before treatment is considered.

On the NHS, Wegovy is currently available through specialist weight management services under NICE guidance, with eligibility criteria that are stricter than the licensed indications and waiting lists that can run long. Private prescribing through a regulated pharmacy is the route for people who meet the licensed criteria but don't qualify for NHS access, or who would rather not wait. The cost of Wegovy privately varies by dose and provider, a separate page covers that context honestly.

Side effects, storage and everyday practicalities

The most commonly reported side effects are gastrointestinal: nausea, diarrhoea, constipation, reflux and burping. These tend to be most noticeable after starting or stepping up to a new dose, and for most people they ease within a week or two. Starting at the lowest dose and titrating gradually is specifically designed to reduce this burden, it is why the schedule exists.

More serious but less common effects include gallbladder problems and, rarely, pancreatitis. The MHRA flagged acute pancreatitis as a known risk in a Drug Safety Update in January 2026: severe stomach pain that spreads to the back, with or without vomiting, warrants urgent medical attention. This applies to all GLP-1 medicines.

On the practical side: Wegovy pens are refrigerated, stored between 2°C and 8°C. Many people keep theirs in the fridge door alongside everyday items and find the weekly injection routine is easy to build into a Monday morning habit. If you need to travel, the Patient Information Leaflet gives exact room-temperature storage windows, always check that before a trip rather than guessing.

If you have questions about interactions with other medicines, our page on taking naproxen alongside semaglutide is one example of where we've addressed specific combination queries. Our general FAQs also cover common concerns. When you're ready to find out whether semaglutide is appropriate for you, the right next step is a proper clinical assessment, speak to our prescribers through a free consultation, and a GPhC-registered Independent Prescriber will review your details the same day. If you're still weighing up whether this treatment is right for you, our page that tells you everything about Wegovy is a good place to start before booking.