STEP-1: Semaglutide 2.4 mg and 14.9% Mean Weight Loss in 68 Weeks

In March 2021, the New England Journal of Medicine published STEP-1 — the pivotal Phase 3 trial that established once-weekly semaglutide 2.4 mg as a weight-loss agent. With 14.9% mean weight reduction over 68 weeks in 1,961 participants, it reset expectations for what pharmacological weight management could achieve.

The Trial

1,961 adults with a BMI ≥30 (or ≥27 with at least one weight-related condition, without type 2 diabetes) were randomised 2:1 to semaglutide 2.4 mg once weekly or placebo, plus lifestyle intervention, for 68 weeks.

Key Results

• 14.9% mean body weight reduction from baseline (vs. 2.4% placebo)
• Mean weight loss of 15.3 kg (33.7 lbs)
• 86% of participants in the semaglutide group achieved ≥5% weight loss vs. 32% on placebo
• 50% achieved ≥15% weight loss vs. 5% on placebo

Improvements in waist circumference, blood pressure, blood glucose, and lipid profiles were also observed.

"The mean weight loss of 14.9% with semaglutide was substantially greater than that seen with previously available pharmacotherapies for obesity." — Wilding et al., NEJM 2021

GLP-1 Mechanism

Semaglutide is a GLP-1 receptor agonist — it mimics glucagon-like peptide-1, a gut hormone released after eating. It activates GLP-1 receptors in the hypothalamus to reduce appetite and food intake, and slows gastric emptying to increase satiety. At 2.4 mg weekly (higher than the diabetes dose of 1 mg), the appetite-suppressing effect is sustained.

Safety

Nausea was reported in 44% of the semaglutide group vs. 16% placebo, mostly mild-to-moderate and transient. Vomiting and diarrhoea were also more common with semaglutide. Serious adverse events occurred at similar rates between groups (9.8% vs. 6.4%), with gallbladder-related events more common with semaglutide — consistent with the GLP-1 class and rapid weight loss.

Historical Context

Before STEP-1, no approved weight-loss drug had demonstrated more than ~10% weight reduction in large trials. Semaglutide's 14.9% result prompted a re-evaluation of obesity treatment, setting the stage for tirzepatide (22.5% in SURMOUNT-1) and retatrutide (24.2% in Phase 2) in the years that followed.