What did the Wegovy STEP 1 trial find — and does it matter to you?

What exactly did the STEP 1 trial set out to test?

STEP 1 was a phase 3, randomised, double-blind, placebo-controlled study funded by Novo Nordisk, the maker of Wegovy. Close to 2,000 adults took part across multiple countries. To qualify, participants needed a BMI of 30 or above, or a BMI of 27 or above alongside at least one weight-related condition such as high blood pressure, raised cholesterol or obstructive sleep apnoea. People with type 2 diabetes were excluded, a separate arm of the STEP programme addressed that population.

Everyone in the trial received lifestyle support: a reduced-calorie diet and guidance on increasing physical activity. Half received weekly semaglutide injections, starting at 0.25mg and titrating upward over 16 weeks to the 2.4mg maintenance dose. The other half received a placebo on the same schedule. At 68 weeks, weight change from baseline was the primary outcome measured.

One misconception worth clearing up: STEP 1 was not testing whether semaglutide helps people lose weight quickly. The trial design was about sustained, medically meaningful reduction over 16 months, not short-term results. The distinction matters when reading the headline figure.

What did the results actually show?

Participants on semaglutide 2.4mg lost an average of around 15% of their starting body weight over the 68-week period. Those on placebo lost around 2.4%. The gap was statistically robust, and the authors reported it in the New England Journal of Medicine in 2021.

Looking beyond the mean tells a fuller story. Around one in three participants on semaglutide lost 20% or more of their body weight. Roughly two-thirds lost at least 10%. A small proportion lost very little. That spread is normal in clinical trials of any medicine, biology, adherence, lifestyle and individual variation all play a role.

Secondary outcomes were also positive: blood pressure, waist circumference and markers of cardiovascular risk improved in the semaglutide group relative to placebo. These secondary findings contributed to why Wegovy later received a UK authorisation not just for weight management but also for reducing major cardiovascular event risk in eligible adults. If you want to explore the broader evidence base for semaglutide, the semaglutide clinical trials overview covers the wider programme.

The trial concluded that at 68 weeks, once semaglutide was stopped, weight regain was observed in follow-up studies, a finding that shaped how prescribers discuss long-term treatment planning with patients. The NICE appraisal of semaglutide (TA875) drew partly on this dynamic when recommending it for a maximum of two years within specialist weight management services on the NHS.

How did side effects look across the 68 weeks?

Most side effects in STEP 1 were gastrointestinal. Nausea was the most commonly reported, followed by vomiting, diarrhoea and constipation. Importantly, these effects were most pronounced in the early weeks (particularly around each dose increase) and generally eased as participants reached and settled on the 2.4mg maintenance dose.

Serious adverse events occurred in roughly 9.8% of the semaglutide group versus 6.4% of placebo. Gallbladder-related events were more frequent in the active group, which is consistent with the known biology of rapid weight loss itself as well as the medicine's effect on gastric motility.

Around 7% of semaglutide participants discontinued because of side effects, compared with around 3% in the placebo group. That discontinuation rate is worth noting: the majority of people in the trial completed treatment, and the GI effects that did occur were usually manageable rather than severe.

For a fuller picture of how semaglutide behaves across the titration schedule, the Wegovy treatment guide covers what to expect at each stage. If you are weighing up the overall semaglutide evidence, the STEP 1 semaglutide page gives additional clinical context, and general information about semaglutide as a medicine is on NHS.uk.

What does the STEP 1 evidence mean for someone considering Wegovy today?

Trial data establishes what a medicine can do under controlled conditions. It does not predict what will happen for any one person. A 15% average conceals a wide distribution, and your own biology, adherence, diet and activity all shift where you land in that range.

What STEP 1 does firmly establish is that semaglutide 2.4mg produces clinically meaningful weight loss at a population level, enough to prompt NICE to recommend it and the MHRA to grant a full UK licence. Those regulatory decisions matter when you are assessing whether a medicine has a credible evidence base.

The trial also reinforces that Wegovy is most effective alongside genuine lifestyle changes. The placebo group also received diet and activity support, and they still lost only around 2.4%. The medicine does a specific job on appetite and gastric emptying; the lifestyle component does a different but complementary job.

For anyone curious about the higher end of the evidence (including what the STEP 4 trial found when treatment continued) there is a separate page covering that data. Pricing and service information, for those at the point of considering private treatment, is set out on the treatment options page. A prescriber reviews every consultation individually before any decision is made. If you think Wegovy could be right for you, you can read more about how a Wegovy trial works before checking your eligibility with our clinical team, and if location is a factor, we also have guidance on finding a Wegovy trial near you.