Introduction
Semaglutide is a glucagon-like peptide-1 (GLP-1) receptor agonist originally developed for the treatment of type 2 diabetes. In 2021, a higher-dose formulation (2.4 mg weekly) received FDA approval for chronic weight management under the brand name Wegovy, based on the landmark STEP (Semaglutide Treatment Effect in People with obesity) clinical trial program.
The STEP program represents one of the most comprehensive clinical development programs for an anti-obesity medication, encompassing multiple phase 3 trials with thousands of participants. This article reviews the key findings from the STEP trials and their implications for the field of weight management.
The STEP Trial Program
The STEP program consisted of multiple randomized, double-blind, placebo-controlled trials evaluating semaglutide 2.4 mg administered once weekly via subcutaneous injection in adults with overweight or obesity. An overview published by Kushner et al. in Obesity summarized the design and key elements of STEP trials 1 through 5 (PubMed: 32441473).
STEP 3: Combined with Behavioral Therapy
The STEP 3 trial, published in JAMA by Wadden et al. (2021), evaluated semaglutide as an adjunct to intensive behavioral therapy (IBT) in 611 adults. Participants receiving semaglutide plus IBT achieved a mean body weight change of -16.0% at 68 weeks, compared to -5.7% with placebo plus IBT. This demonstrated that semaglutide provides substantial additional weight loss even when combined with the most intensive behavioral intervention available (PubMed: 33625476).
STEP 4: Weight Maintenance
A critical question for any weight loss therapy is whether the benefits are maintained over time. The STEP 4 trial addressed this by first treating all participants with semaglutide for 20 weeks, then randomizing them to continue treatment or switch to placebo. Published in JAMA by Rubino et al. (2021), the results showed that those who continued semaglutide maintained their weight loss, while those switched to placebo experienced significant weight regain, confirming the need for ongoing treatment (PubMed: 33755728).
STEP 5: Two-Year Data
The STEP 5 trial provided the longest-duration data in the program. Published in Nature Medicine by Garvey et al. (2022), this 104-week study showed that 77.1% of participants on semaglutide achieved at least 5% weight loss at two years, compared to 34.4% with placebo. The sustained efficacy over two years was a significant finding for the field (PubMed: 36216945).
STEP 8: Head-to-Head Comparison
The STEP 8 trial directly compared weekly semaglutide 2.4 mg to daily liraglutide 3.0 mg (the previously approved GLP-1 RA for weight management). Published in JAMA by Rubino et al. (2022), semaglutide produced significantly greater weight loss (-15.8%) than liraglutide (-6.4%) at 68 weeks, establishing semaglutide as the more effective option (PubMed: 35015037).
Weight Loss Outcomes Summary
Across the 68-week STEP trials in participants without diabetes, the weight loss outcomes were consistent and substantial:
- Mean weight loss of 14.9-17.4% with semaglutide 2.4 mg
- 86-89% of participants achieved at least 5% weight loss
- 69-79% achieved at least 10% weight loss
- 51-64% achieved at least 15% weight loss
- 32-40% achieved at least 20% weight loss
Mechanism of Action
Semaglutide works by mimicking GLP-1, an incretin hormone released after eating. It activates GLP-1 receptors in the hypothalamus to reduce appetite and food intake, slows gastric emptying to increase satiety, and enhances insulin secretion in a glucose-dependent manner. The 2.4 mg dose used for weight management is higher than the 1.0 mg dose used for type 2 diabetes.
Safety Profile
Gastrointestinal adverse events were the most commonly reported side effects across the STEP program, occurring in approximately 82.2% of semaglutide-treated participants versus 53.9% on placebo. These were mostly mild to moderate and included nausea, diarrhea, vomiting, and constipation. Most GI events occurred during dose escalation and tended to resolve over time.
Summary
The STEP clinical trial program established semaglutide 2.4 mg as a highly effective weight management therapy, consistently producing weight losses of 15-17% over 68 weeks with sustained efficacy at two years. Its superiority over liraglutide was demonstrated in a head-to-head trial. The data represent the most robust clinical evidence for any anti-obesity peptide therapy to date, with clear dose-response relationships and well-characterized safety profiles.