One 68-week placebo-controlled trial became the evidence base for Wegovy's FDA approval — and for nearly every weight-loss claim made about semaglutide since. Here's what STEP 1 actually measured.
STEP 1 (NCT03548935) was a phase 3, randomized, double-blind, placebo-controlled trial run at 129 sites across 16 countries in Asia, Europe, and North and South America. It enrolled 1,961 adults with a BMI of 30 or greater, or 27 or greater with at least one weight-related condition, who did not have diabetes.
Participants were randomized 2:1 to once-weekly subcutaneous semaglutide at a maintenance dose of 2.4mg, or a matching placebo — both on top of the same lifestyle intervention: counseling every four weeks, a reduced-calorie diet, and a target of 150 minutes of activity per week. Dosing started at 0.25mg and stepped up every four weeks to reach 2.4mg by week 16, the same escalation pattern semaglutide uses for its other indications.
The primary endpoints were percent change in body weight and the proportion of participants reaching at least 5% weight loss, both measured at week 68.
Semaglutide beat placebo on every efficacy measure the trial tracked, by a wide margin.
About one in three participants on semaglutide lost at least 20% of their starting body weight — a magnitude the trial authors compared to what's typically seen one to three years after sleeve gastrectomy surgery. Waist circumference, blood pressure, and markers like fasting glucose and C-reactive protein also improved more on semaglutide than placebo.
Gastrointestinal side effects — nausea, diarrhea, vomiting, and constipation — were the most common adverse events, reported by 74.2% on semaglutide versus 47.9% on placebo, mostly during the dose-escalation period. 7.0% of the semaglutide group discontinued treatment due to adverse events, versus 3.1% on placebo.
On June 4, 2021, the FDA approved semaglutide 2.4mg as Wegovy for chronic weight management in adults with obesity, or overweight with at least one weight-related condition — the first new weight-loss drug approval since 2014, built directly on the STEP 1 results.
Sources: Wilding JPH, Batterham RL, Calanna S, et al., New England Journal of Medicine, 2021 (STEP 1, NCT03548935, doi:10.1056/NEJMoa2032183) · AJMC trial summary, Feb 10, 2021. NEJM full text.
The 2.4mg dose is the Wegovy dose, not the Ozempic dose. Ozempic, semaglutide's diabetes brand, tops out at 2mg and isn't approved or dosed for this magnitude of weight loss — Wegovy's titration schedule and top maintenance dose exist specifically to reach 2.4mg.
The trial population skewed White (75.1%) and female (74.1%), with a mean age of 46. The authors themselves flagged this as a limitation — individual results outside that population may vary, and STEP 1 alone can't answer how the drug performs across every demographic.
GI side effects during titration are the norm, not a sign something's wrong. Nearly three-quarters of the semaglutide group in STEP 1 reported them, mostly early and mostly mild to moderate. Knowing that ahead of time — and when to flag symptoms to a prescriber — makes the escalation period easier to get through.
This article summarizes a single published clinical trial and a contemporaneous news summary for general education. It is not medical advice. Individual results vary, and any decision about starting, adjusting, or stopping semaglutide should be made with a prescriber.