What the STEP 1 study found about semaglutide and weight loss

You've just read the headline figure, here's what was actually tested

Imagine reading that a medicine produced 15% average weight loss and wondering whether that applies to you. It's a fair question, and the trial design matters a lot here. STEP 1 enrolled 1,961 adults who had a BMI of 30 or above, or a BMI of 27 or above alongside at least one weight-related health condition such as raised blood pressure, high cholesterol, obstructive sleep apnoea or cardiovascular disease. People with type 2 diabetes were excluded, that population was studied separately.

Participants were randomly assigned to receive semaglutide 2.4 mg weekly or a placebo injection, and both groups received counselling on diet and activity throughout. The dose wasn't 2.4 mg from day one: it started low and was titrated upward over roughly 16 weeks, giving the body time to adjust. After 68 weeks, the semaglutide group had lost an average of 15.3% of their starting weight; the placebo group lost 2.6%. More striking still, 50.5% of those on semaglutide lost at least 15% of their body weight, a result that had rarely been seen with a medicine outside of surgical settings.

The semaglutide STEP 1 results are the bedrock on which Wegovy's UK weight-management licence was built, and they're the figures NICE cited when evaluating the treatment. That context matters when you're deciding how to read any headline you see.

What the trial revealed about side effects at this dose

No medicine produces results without trade-offs, and STEP 1 documented semaglutide's side-effect profile in detail. Gastrointestinal effects (nausea, vomiting, diarrhoea, constipation, stomach discomfort) were the most frequently reported, affecting a meaningful proportion of participants. Crucially, most of these events were mild to moderate in severity and were concentrated in the early weeks of treatment, particularly during dose increases. By the time participants reached and settled on the 2.4 mg maintenance dose, many reported that symptoms had largely resolved.

About 4.5% of people on semaglutide discontinued treatment due to adverse events, compared with 0.8% on placebo. Serious adverse events were uncommon in both groups. The NHS's semaglutide medicine guide sets out the full side-effect picture in practical terms, including signs that warrant medical advice.

One finding worth knowing: participants who stopped semaglutide after the trial regained a substantial proportion of the weight they had lost within a year. This wasn't a failure of the medicine, it reflected that semaglutide works on appetite-regulating pathways that only remain active while the drug is present. It's one reason why Wegovy's prescribing framework involves ongoing clinical review rather than a one-off prescription.

If you have specific questions about your health history and how it relates to GI side effects, that conversation belongs with a prescriber, not a search engine.

How STEP 1 fits into the wider STEP programme, and what came next

STEP 1 was the first in a series of trials. The programme went on to study semaglutide in people with type 2 diabetes, in people who had already lost weight and were trying to maintain it, and in populations with cardiovascular risk. Each trial answered a different question about the same medicine. STEP 2 examined the diabetes population; STEP 3 added intensive behavioural therapy. Together they built the clinical case that led regulators to grant Wegovy its licence.

Since then, the evidence base has grown further. Head-to-head data from the SURMOUNT-5 trial, published in the New England Journal of Medicine in 2025, compared semaglutide 2.4 mg directly with tirzepatide 15 mg and found greater average weight loss with tirzepatide, a finding referenced by NICE's tirzepatide appraisal (TA1026). That doesn't diminish STEP 1's importance; it simply means the evidence landscape has expanded. Which medicine suits any individual is a clinical question, shaped by medical history, tolerability and personal preference.

For a broader view of how all the STEP studies relate to each other, the STEP trial overview covers the programme as a whole. If you're weighing up semaglutide against other options, semaglutide's treatment page is a useful starting point before a consultation.

From trial data to a prescription: what the process actually looks like

Reading the trial results is one thing; understanding what getting treatment involves is another. Wegovy is available in the UK on private prescription through pharmacies registered with the GPhC. At nume, you submit a free consultation online. A GPhC-registered prescriber reads your answers personally and, if treatment is clinically appropriate, issues a prescription the same day. Worth knowing on cost: the price context for Wegovy in the UK is worth understanding before you start, because a legitimate prescription price always includes clinical assessment, that's not a hidden add-on, it's the whole point.

Once approved, your treatment is dispatched the same day (for orders placed before noon, Monday to Friday) and arrives the next working day via DPD. It comes in plain, unbranded packaging, just a tracked parcel that gives nothing away. Inside, the Wegovy pen is pre-filled and ready to use once your prescriber has walked you through the first injection. Before each repeat order, your case is reviewed again. There are no auto-renewals, no subscriptions.

If you're ready to discuss whether semaglutide is right for your situation, you can check your eligibility with a free consultation, a prescriber, not an algorithm, will review it.