What the clinical studies on Wegovy tell us — and what they don't

The most common misreading: trial averages are not personal predictions

The figure that travels fastest is roughly 15% average weight loss over 68 weeks. That number is real, it comes from STEP 1, which randomised 1,961 adults with obesity or overweight plus at least one weight-related condition, none with type 2 diabetes. But an average conceals a distribution. Some participants lost considerably more; others lost less. Around a third of those on semaglutide in STEP 1 lost more than 20% of their starting weight. A smaller proportion lost under 5%. Treating the average as a floor, or as a guarantee for any individual, is the single biggest misreading you will find when reading through the wegovy studies on weight loss.

The design matters too. Every participant was given structured lifestyle support alongside the injection. The medicine reduced appetite and slowed gastric emptying; the lifestyle programme gave people the framework to act on that reduced appetite. Separating the two in real-world use changes outcomes in ways the trials were not designed to measure. A closer look at what STEP 1 actually measured unpacks the trial design in plain terms if you want the detail.

One practical note from our prescribers: people sometimes arrive expecting a certain number because they read the headline. The evidence is strong (genuinely strong) but it describes a population, not a promise to an individual. Your prescriber will say the same.

What the STEP programme actually covered across its four trials

Wegovy's approval rested on the STEP (Semaglutide Treatment Effect in People with Obesity) programme, four phase-3 randomised controlled trials published between 2021 and 2022. STEP 1 tested the 2.4 mg dose against placebo in adults with obesity or overweight and a weight-related condition. STEP 2 focused on adults with type 2 diabetes, a population where the response was meaningful but more modest, averaging around 10% loss, because diabetes itself affects how the body responds to GLP-1 stimulation. STEP 3 added intensive behavioural therapy to the standard lifestyle support; average losses there approached 16%. STEP 4 tested what happened when participants who had already lost weight on semaglutide either continued or switched to placebo, those who switched regained most of what they had lost within a year, which tells us something important about how long treatment may need to continue.

Taken together, the semaglutide studies that make up STEP give regulators and prescribers a reasonably complete picture across different patient types. The STEP 1 trial specifically remains the most-cited because its population most closely matches the licensed weight-management indication. NICE's appraisal of semaglutide (TA875) drew heavily on this programme when recommending it for use within specialist weight management services.

Beyond weight: what the cardiovascular and longer-term Wegovy research found

The SELECT trial, reported in 2023, took the semaglutide evidence in a different direction. It enrolled over 17,000 adults with established cardiovascular disease and overweight or obesity but no diabetes, and followed them for a median of just over three years. Semaglutide 2.4 mg reduced the risk of major cardiovascular events (heart attack, stroke, or cardiovascular death) by around 20% compared with placebo. This was the first weight-loss medicine to show such an outcome in a trial of this size and duration. The UK regulator subsequently approved an additional licence indication for semaglutide covering cardiovascular risk reduction in eligible adults.

For readers interested in the longer picture, what long-term semaglutide research tells us covers SELECT and the durability data in more depth. The short version: weight lost tends to return when treatment stops, and cardiovascular benefit appears to extend beyond weight loss alone, though the mechanisms are still being studied. It is an evolving evidence base, and new semaglutide studies continue to emerge.

The full overview of Wegovy as a licensed UK treatment sets this research in the context of how the medicine is actually used in practice.

How the evidence base shapes private and NHS access in the UK today

NICE used the STEP programme and SELECT data to recommend semaglutide (Wegovy) for NHS use under TA875, but with specific conditions: a maximum of two years, within a specialist weight management service, for adults meeting defined BMI and comorbidity criteria. Those restrictions reflect both budget considerations and the current evidence on long-term use rather than a doubt about the medicine's effectiveness. NHS waiting lists for specialist services can be long.

Private prescribing does not carry the two-year NHS cap, but the clinical assessment is no less thorough. At nume, a GPhC-registered Independent Prescriber reads every consultation personally (the same day you submit it) before any decision is made. If you are weighing up what Wegovy costs privately against the NHS route, the relevant question is not just price but how the two pathways compare in terms of access, speed and clinical oversight.

Semaglutide is a prescription-only medicine. The strength of the trial evidence does not change that, and it does not change the need for a proper clinical review before starting. If you want to understand whether the evidence applies to your situation, speaking to our prescribers is the straightforward next step.