STEP-1

STEP 1 is the trial that made semaglutide a household name. Published in The New England Journal of Medicine in 2021, it randomized 1,961 adults with obesity, or with excess weight plus a weight-related condition, and no diabetes, to once-weekly semaglutide 2.4 mg or placebo for 68 weeks. Everyone also received lifestyle support.

The headline result: average body weight fell 14.9% on semaglutide versus 2.4% on placebo, a treatment difference of 12.4 percentage points. In absolute terms that was 15.3 kg versus 2.6 kg. The proportions matter as much as the average — 86.4% of the semaglutide group lost at least 5% of body weight, 69.1% lost at least 10%, and 50.5% lost at least 15%, against 31.5%, 12.0% and 4.9% on placebo.

It is worth sitting with what those numbers do and do not say. Half the treated group lost 15% or more; the other half did not. Roughly one in seven people on semaglutide failed to reach even 5%. Nausea and diarrhea were the most common adverse events, usually mild to moderate and fading with time, and 4.5% of the semaglutide group stopped treatment because of gastrointestinal events versus 0.8% on placebo.

The most frequent misreading is treating 14.9% as a promise. It is a group average from a supported trial population that received structured lifestyle counseling throughout. Real-world results skew lower, partly because adherence and dose escalation are harder outside a trial. The second misreading is assuming the result persists after stopping — STEP 1 did not test that, and the trials that did found most of the weight returns.

Useful questions: what percentage loss would count as success for your health goals rather than a number on a scale, and how your prescriber will decide whether the drug is working by week 12 or 16. Our trial side-effect review and weight-loss projection tool put the figures in context.