The STEP Trial Program: Semaglutide for Weight Management

The STEP program comprised four concurrent Phase 3 randomised, double-blind, placebo-controlled trials enrolling a total of approximately 4,500 participants across STEP 1, STEP 2, STEP 3, and STEP 4. STEP 1 enrolled individuals with obesity (BMI ≥30 kg/m²) or overweight (BMI ≥27 kg/m²) with weight-related comorbidities such as hypertension, dyslipidaemia, or cardiovascular disease. STEP 2 specifically enrolled participants with type 2 diabetes, while STEP 3 and STEP 4 enrolled participants with chronic weight management needs. The trials employed identical primary endpoints and study designs, enabling robust meta-analytical evaluation of semaglutide's weight loss efficacy across diverse populations.

All STEP trials used once-weekly subcutaneous semaglutide (escalated from 0.4 mg to 2.4 mg) versus placebo over 68 weeks of treatment, followed by a 20-week follow-up observation period. This design was chosen to mirror regulatory guidance on obesity treatment development and to establish both short-term efficacy and durability of effect.

STEP 1 enrolled 1,961 participants (mean BMI 37.6 kg/m²) with obesity/overweight and comorbidities; STEP 2 enrolled 1,210 participants with type 2 diabetes (mean BMI 32.3 kg/m²); STEP 3 enrolled 611 participants with obesity requiring inpatient weight loss treatment; STEP 4 enrolled 902 participants with cardiovascular disease. Baseline characteristics varied across trials to represent real-world patient populations with differing metabolic phenotypes and disease burdens. The trials excluded individuals with personal or family history of thyroid cancer or multiple endocrine neoplasia (a regulatory precaution based on rodent carcinogenicity signals), pregnant or nursing individuals, and those with severe renal or hepatic impairment.

The dose-escalation schedule began with 0.4 mg weekly for 4 weeks, increasing to 0.8 mg, then 1.2 mg, 1.6 mg, 2.0 mg, and finally 2.4 mg (target dose) for the maintenance phase. Participants with intolerance to escalation were permitted to continue on lower tolerated doses. This gradual escalation was designed to improve tolerability and reduce gastrointestinal side effects that had plagued earlier GLP-1 agonist development. Concomitant lifestyle counselling (dietary and exercise guidance) was provided to all participants in both arms.

The primary endpoint across all STEP trials was percentage change in body weight from baseline to week 68. STEP 1 demonstrated a mean weight loss of 15.3% with semaglutide 2.4 mg versus 2.6% with placebo (difference 12.7 percentage points; p<0.001). STEP 2 (in type 2 diabetes) showed mean weight losses of 10.5% with semaglutide versus 3.2% with placebo (difference 7.3 percentage points). STEP 3 and STEP 4 showed consistent efficacy across their respective populations. Across the entire STEP program, mean weight loss with semaglutide 2.4 mg ranged from 10.5% to 15.3% depending on baseline characteristics, substantially exceeding placebo responses in all populations.

Beyond mean weight loss, the STEP trials quantified weight loss response rates. In STEP 1, 86% of participants receiving semaglutide achieved ≥5% weight loss (clinical threshold for metabolic benefit) compared to 31% on placebo. Furthermore, 69% achieved ≥10% weight loss and 35% achieved ≥15% weight loss—response rates not observed with prior weight loss medications. These high response rates demonstrate semaglutide's superior efficacy among available pharmacological weight management tools.

Secondary endpoints in the STEP trials included changes in cardiometabolic risk factors, physical function, and quality of life. Participants receiving semaglutide showed consistent improvements in fasting glucose, HbA1c (in those with diabetes), systolic and diastolic blood pressure (approximately 2–3 mmHg reduction), LDL cholesterol, and triglycerides. STEP 2, enrolling participants with type 2 diabetes, demonstrated HbA1c reductions of approximately 1.1–1.5 percentage points with semaglutide, exceeding most oral diabetes medications. These cardiometabolic improvements exceeded those expected from weight loss alone, suggesting direct metabolic benefits of GLP-1 receptor agonism independent of weight reduction.

Physical function assessments using the 6-minute walk test showed meaningful improvements with semaglutide, with participants demonstrating increased walking distance and reduced dyspnoea. Quality of life measures (using validated instruments such as the EQ-5D) showed improvements with semaglutide compared to placebo. Notably, the degree of weight loss correlated with improvements in physical function and quality of life, supporting the clinical relevance of the weight loss achieved.

Gastrointestinal adverse events (nausea, vomiting, diarrhoea, constipation) were the most common side effects, occurring in 25–40% of semaglutide-treated participants compared to approximately 10% with placebo. Most gastrointestinal events were mild to moderate and transient, resolving within 4–8 weeks despite continued treatment. Serious gastrointestinal events (acute pancreatitis, gallbladder-related events) were rare and occurred at similar frequencies between semaglutide and placebo arms, not suggesting a causal relationship.

The STEP trials detected no increased incidence of thyroid cancer, thyroid nodules, or other malignancies with semaglutide versus placebo. Heart rate increases of 2–3 bpm were observed with semaglutide but did not translate into increased cardiovascular events. Notably, the SELECT trial (discussed separately) in non-diabetic obesity populations reported cardiovascular benefits, not harms. These safety findings supported regulatory approval of semaglutide for weight management despite the preclinical thyroid toxicity signal.

The STEP trials enrolled relatively well-selected populations with good medical access and adherence; real-world weight loss may be lower due to higher discontinuation rates from side effects, cost barriers, and lower adherence. The STEP population was predominantly white and middle-to-upper income, limiting generalisability to more diverse populations. The 68-week duration, while substantial, does not capture long-term (>2 years) efficacy and durability. Additionally, the trials provided intensive lifestyle counselling alongside pharmacotherapy; the relative contribution of counselling versus semaglutide to observed weight loss was not rigorously quantified.

The STEP trials excluded individuals with prior bariatric surgery, severe renal/hepatic disease, and personal/family history of thyroid cancer, limiting applicability to these populations. Furthermore, the trials did not directly address whether semaglutide could achieve and sustain weight loss in the most severely obese populations (BMI >50 kg/m²). Despite these limitations, the STEP program provided robust evidence that semaglutide is highly efficacious for weight management across diverse clinical populations, setting a new standard for obesity pharmacotherapy.