What the semaglutide clinical trials found — and what it means for you

You've read the headlines, here's what the trials were actually measuring

Imagine you've been reading about Wegovy online for a few weeks. The numbers keep appearing: 15%, 68 weeks, placebo-controlled. What do those phrases mean in real terms?

The STEP 1 trial enrolled 1,961 adults who had a BMI of 30 or above, or 27 or above with at least one weight-related health condition such as high blood pressure or high cholesterol, but no type 2 diabetes. Half received semaglutide 2.4mg once weekly, titrated gradually over roughly 16 weeks; the other half received a placebo injection. Both groups followed a reduced-calorie diet and were encouraged to be more active. After 68 weeks, the semaglutide group had lost an average of around 15% of their starting body weight. The placebo group lost around 2.4%. That gap (roughly 12.5 percentage points) is what clinicians call the treatment effect.

It is worth being clear about what an average means. Some participants lost considerably more than 15%; others less. Roughly a third of participants on semaglutide achieved more than 20% weight loss. Around 86% lost at least 5% of their body weight, a threshold associated with clinically meaningful improvements in blood pressure, cholesterol and blood sugar. If you want a closer look at how those outcomes were measured and what they mean for people starting treatment today, our guide to semaglutide clinical trial weight loss walks through the evidence in detail. The trials were not measuring how people felt about their appearance; they were measuring health outcomes. That distinction shapes how the medicine is licensed and prescribed in the UK today.

Semaglutide belongs to a class of medicines called GLP-1 receptor agonists. It mimics a gut hormone that signals fullness to the brain and slows gastric emptying. Those mechanisms were already understood from diabetes trials; the STEP programme was the first to test the 2.4mg weight-management dose systematically at scale.

What happened in the later STEP trials, and why the 7.2mg dose matters

STEP 1 was followed by several companion trials. STEP 2 enrolled adults with type 2 diabetes alongside obesity (a population where the medicine was already used) and found average weight loss of around 10% at 2.4mg. STEP 3 added intensive behavioural support and produced higher average losses. STEP 4 looked at what happened when semaglutide was stopped: weight largely returned over the following year, reinforcing that these medicines work while taken rather than providing a permanent reset.

The more recent development is the 7.2mg dose. The MHRA approved a dedicated single-dose 7.2mg Wegovy pen on 14 April 2026 for adults with a BMI of 30 or above. Trial data associated with that dose reported average weight loss of around 20.7% over 72 weeks, which begins to approach the results seen with tirzepatide in its own programme. Our overview of the 7.2mg semaglutide trial goes through those figures in more detail.

For a broader look at how semaglutide's results compare across the full trial series, the Wegovy clinical trial results page sets out the numbers side by side. What the later trials confirmed collectively is that the dose reached matters, adherence matters, and stopping the medicine reverses most of the benefit. Those are the practical findings that inform how prescribers set expectations before treatment begins.

What the trials mean for accessing treatment in the UK

NICE relied heavily on the STEP programme when evaluating whether semaglutide should be recommended on the NHS. Their appraisal of semaglutide for weight management, NICE technology appraisal TA875, recommends it within specialist weight management services, for a maximum of two years, for adults with a BMI of 35 or above and at least one weight-related condition. Ethnic-background adjustments lower that BMI threshold by 2.5 kg/m² for certain groups. NHS waiting lists for those services remain long in many areas.

Private prescription is the other route. If you're thinking about this around payday or at the start of a new month when motivation tends to run high, it helps to know that a private consultation can begin on any working day; an order placed before 12pm goes out the same day once clinically approved. That does not change what the trials found, but it does mean the timeline from reading this page to having treatment in your fridge is often shorter than people expect.

Semaglutide injections for weight management are sold in the UK under the brand name Wegovy. Our Wegovy treatment page explains the full process. If you want to understand what the buying journey actually involves before committing to a consultation, the guide to buying Wegovy is a practical place to start.

One note that often surprises people: Ozempic is also semaglutide, but it is licensed in the UK for type 2 diabetes, not weight management. The trials that produced the 15% figure used the Wegovy formulation at the 2.4mg dose. Using Ozempic for weight loss is outside its UK licence, and any prescriber offering it for that purpose should be asked to explain why. That distinction matters both clinically and legally.

Side effects seen in the trials, and the signal that led to a 2026 MHRA update

The side-effect profile in the STEP trials was dominated by gastrointestinal symptoms: nausea, diarrhoea, vomiting, constipation and indigestion were the most reported. These were most common in the weeks after starting or after a dose increase, and most settled as the body adjusted. Around 7% of participants in STEP 1 discontinued because of side effects, compared with around 3% on placebo, a meaningful but minority figure.

Across the wider GLP-1 class, acute pancreatitis emerged as a known but infrequent serious side effect. In January 2026 the MHRA issued a Drug Safety Update specifically addressing this risk for GLP-1 medicines, advising people to seek urgent medical attention for severe, persistent stomach pain that reaches into the back, with or without vomiting. That update did not change the overall benefit-risk assessment for semaglutide, but it reinforced the importance of clinical oversight throughout treatment rather than treating these medicines as set-and-forget.

Anyone who experiences suspected side effects can report them through the MHRA Yellow Card scheme. Our page on what the Wegovy clinical trials covered includes a fuller breakdown of how side effects were measured and reported across the programme. Questions about your own side effects or how to manage them should always go to your prescriber first.