Semaglutide and ~15% weight loss: what the STEP 1 trial showed

STEP 1 was the trial that brought semaglutide into the obesity conversation in earnest. Published in the New England Journal of Medicine in 2021, it was a randomised, double-blind, placebo-controlled study of 1,961 adults who were overweight or obese but did not have diabetes. Participants were assigned in a 2:1 ratio to either once-weekly semaglutide, titrated up to 2.4 mg, or a matching placebo. Both groups also received lifestyle support — counselling on diet and physical activity — so the medication was tested as an addition to behavioural change rather than as a replacement for it.

The trial ran for 68 weeks. The headline result was a mean reduction in body weight of about 14.9% in the semaglutide group, compared with roughly 2.4% in the placebo group — a treatment difference of around 12 percentage points. Put plainly, the average person on the medication lost close to a seventh of their starting weight over roughly sixteen months.

What was notable was not only the magnitude but the proportions reaching meaningful thresholds. About 86% of the semaglutide group lost at least 5% of their body weight, against roughly 32% on placebo. A substantial share reached 10%, 15%, and even 20% loss. The most common side effects were gastrointestinal — nausea and diarrhoea — generally mild to moderate and tending to settle over time, though they led a small minority to stop treatment.

Before STEP 1, weight-loss medications had generally delivered modest results — single-digit percentage reductions that often disappointed in practice. The scale of effect seen here moved the conversation closer to figures previously associated only with bariatric surgery, and it has shaped how obesity medicine now frames pharmacological treatment.

Two caveats deserve emphasis. First, the weight loss was achieved while taking the medication: the trial's own extension found that, after participants stopped, much of the lost weight returned over the following year — consistent with the view that obesity behaves as a chronic, relapsing condition. Second, averages conceal wide individual variation; some participants lost a great deal, others relatively little. STEP 1 establishes what is possible at the population level, not what any one person should expect. None of this is medical advice — decisions about treatment belong with a qualified clinician who can weigh the benefits against the side-effect profile and the likely need for ongoing therapy.