STEP 1 Trial: Semaglutide 2.4 mg Results, Methods, and What Every Patient Should Know

What Was the STEP 1 Trial and Why Does It Matter?

STEP 1 is the registration trial that produced the evidence base for Wegovy's FDA approval. Published in the New England Journal of Medicine in February 2021, it enrolled 1,961 adults with a BMI of 30 or greater (or BMI <30 with at least one weight-related comorbidity), all without type 2 diabetes, and randomized them 2:1 to subcutaneous semaglutide 2.4 mg once weekly or placebo for 68 weeks alongside a reduced-calorie diet and increased physical activity counseling [1].

The primary endpoints were the percentage change in body weight from baseline to week 68 and the proportion of participants achieving at least 5% weight loss. Semaglutide met both with high statistical significance (P<0.001 for both). Mean weight loss was 14.9% in the semaglutide group versus 2.4% in the placebo group. A full 69.1% of semaglutide participants lost 10% or more of their body weight, and 50.5% lost 15% or more.

Beyond the headline number, STEP 1 also documented meaningful improvements in waist circumference (down 13.54 cm with semaglutide vs. 4.13 cm with placebo), blood pressure, and fasting plasma glucose. Those secondary results gave regulators and clinicians reason to view semaglutide 2.4 mg as a metabolic therapy, not merely a weight-loss tool.

The most common adverse events were gastrointestinal: nausea affected 44.2% of semaglutide participants, vomiting 24.5%, and diarrhea 29.7%, though most events were mild to moderate and transient [1]. Discontinuation due to adverse events was 7.0% in the semaglutide group versus 3.1% with placebo.

STEP 2: Semaglutide in Adults With Type 2 Diabetes

STEP 2 asked a different question: how does semaglutide 2.4 mg perform in people who already have type 2 diabetes, a population where GLP-1 agonists were already used at lower doses for glycemic control?

Published in The Lancet in March 2021, STEP 2 (N=1,210) randomized adults with type 2 diabetes and overweight or obesity to semaglutide 2.4 mg, semaglutide 1.0 mg, or placebo for 68 weeks [2]. Mean body weight reduction was 9.6% with semaglutide 2.4 mg, 7.0% with semaglutide 1.0 mg, and 3.4% with placebo (P<0.001 for both active doses vs. placebo). HbA1c fell by 1.6 percentage points in the 2.4 mg group versus 0.4 percentage points with placebo.

The weight loss in STEP 2 was meaningfully lower than in STEP 1, a well-documented finding attributed to the anti-obesity effects of existing diabetes medications and to altered metabolic physiology in longstanding type 2 diabetes. Clinicians should set realistic expectations with diabetic patients: 9 to 10% body weight loss is still clinically significant and exceeds what most oral agents achieve, but it is not the 15% seen in STEP 1.

STEP 3: Intensive Behavioral Therapy Added to Semaglutide

STEP 3 tested whether combining semaglutide 2.4 mg with intensive behavioral therapy (IBT) would produce greater weight loss than semaglutide with standard dietary advice. Published in JAMA in April 2021, STEP 3 enrolled 611 adults without type 2 diabetes and randomized them 2:1 to semaglutide plus IBT versus placebo plus IBT for 68 weeks [3].

Mean weight loss reached 16.0% in the semaglutide group versus 5.7% in the placebo group (P<0.001). The 16% figure exceeded STEP 1's 14.9%, suggesting that structured behavioral support adds roughly 1 to 1.5 percentage points of additional weight loss over standard lifestyle counseling. The JAMA trial also noted that 75.3% of semaglutide participants achieved at least 10% weight loss, compared with 27.0% in the placebo-plus-IBT arm.

This matters clinically. Patients who combine a structured behavioral program with semaglutide 2.4 mg may lose close to one-sixth of their starting body weight over 16 months.

STEP 5: Two-Year Data and What Happens Over Time

Most obesity trials run 52 to 68 weeks. STEP 5 extended treatment to 104 weeks (two years) to answer whether weight loss is maintained or erodes with continued semaglutide use.

Published in Nature Medicine in October 2022, STEP 5 (N=304) enrolled adults with BMI <27 and at least one comorbidity or BMI 30 or higher, all without type 2 diabetes, and randomized them 1:1 to semaglutide 2.4 mg or placebo for 104 weeks [4]. Mean weight loss at week 104 was 15.2% with semaglutide versus 2.6% with placebo (P<0.001). Body weight continued to decrease through approximately week 60 and then plateaued, with no meaningful regain between weeks 60 and 104 in those who remained on treatment.

The two-year data carry an important implication: semaglutide does not produce a "ceiling" effect in the first year followed by failure in the second. Patients who tolerate the drug and remain adherent can sustain clinically meaningful weight loss for at least 24 months.

The STEP-5 safety profile at 104 weeks was consistent with STEP 1. Serious adverse events were reported in 9.8% of the semaglutide group versus 6.4% for placebo, and no new safety signals emerged beyond those already identified in the 68-week trials [4].

STEP 8: Semaglutide 2.4 mg Head-to-Head Against Liraglutide 3.0 mg

Before semaglutide 2.4 mg (Wegovy) entered clinical practice, liraglutide 3.0 mg (Saxenda) was the only GLP-1 agonist approved specifically for chronic weight management. STEP 8 provided the first head-to-head comparison.

Published in JAMA in January 2022, STEP 8 (N=338) randomized adults without type 2 diabetes to once-weekly semaglutide 2.4 mg or once-daily liraglutide 3.0 mg for 68 weeks, both alongside lifestyle intervention [5]. Mean weight loss was 15.8% with semaglutide versus 6.4% with liraglutide (treatment difference: 9.4 percentage points, P<0.001). Semaglutide participants were also more likely to achieve the 10% and 20% weight-loss thresholds.

Gastrointestinal adverse events occurred in both groups at similar rates (84.1% semaglutide, 82.7% liraglutide), though discontinuation due to GI events was lower with semaglutide (3.6% vs. 6.6%). The convenience advantage of once-weekly versus once-daily dosing, combined with nearly 2.5-fold greater weight loss, explains why liraglutide 3.0 mg prescribing dropped sharply after STEP 8 was published.

How the STEP Trials Compare: A Side-by-Side Summary

The five STEP trials address distinct clinical questions. Reading them together gives a more complete picture than any single trial alone.

| Trial | N | Population | Duration | Sema weight loss | Placebo weight loss | |---|---|---|---|---|---| | STEP 1 | 1,961 | No T2D, BMI <30+ | 68 weeks | 14.9% | 2.4% | | STEP 2 | 1,210 | T2D, overweight/obese | 68 weeks | 9.6% | 3.4% | | STEP 3 | 611 | No T2D, + intensive IBT | 68 weeks | 16.0% | 5.7% | | STEP 5 | 304 | No T2D, BMI <30+ | 104 weeks | 15.2% | 2.6% | | STEP 8 | 338 | No T2D, vs liraglutide | 68 weeks | 15.8% | 6.4% (lira) |

Across the non-diabetic populations (STEP 1, 3, 5, 8), mean weight loss with semaglutide 2.4 mg ranged from 14.9% to 16.0%, a range narrow enough to guide patient counseling with confidence. In patients with type 2 diabetes, clinicians should benchmark expectations at approximately 9 to 10%.

The SELECT Trial: Cardiovascular Outcomes Beyond Weight Loss

Weight loss was never the only metric that mattered to cardiologists. The SELECT trial (Semaglutide Effects on Cardiovascular Outcomes in People with Overweight or Obesity) enrolled 17,604 adults with pre-existing cardiovascular disease, BMI <27, and no history of diabetes. Participants were randomized to semaglutide 2.4 mg or placebo and followed for a median of 39.8 months.

Published in the New England Journal of Medicine in November 2023, SELECT showed a 20% relative risk reduction in major adverse cardiovascular events (MACE: cardiovascular death, non-fatal myocardial infarction, non-fatal stroke) with semaglutide versus placebo (HR 0.80 to 95% CI 0.72 to 0.90, P<0.001) [6]. That benefit accrued in patients who were already on guideline-directed medical therapy including statins, aspirin, and beta-blockers.

The SELECT investigators noted: "In this trial, weekly subcutaneous semaglutide 2.4 mg led to a significantly lower incidence of death from cardiovascular causes, nonfatal myocardial infarction, or nonfatal stroke than placebo among patients with preexisting cardiovascular disease and overweight or obesity but without diabetes" [6]. That finding shifted how cardiologists view GLP-1 therapy: not as adjunctive weight management, but as primary cardiovascular risk reduction.

How Does Semaglutide Compare to Tirzepatide Across the SURMOUNT Program?

Tirzepatide (Zepbound, Mounjaro) is a dual GIP/GLP-1 receptor agonist that competes directly with semaglutide in the chronic weight management space. The SURMOUNT program mirrors the structure of STEP trials.

SURMOUNT-1 (N=2,539, no type 2 diabetes) showed mean weight loss of 20.9% with tirzepatide 15 mg at 72 weeks versus 3.1% with placebo (P<0.001) [7]. SURMOUNT-2 (N=938, type 2 diabetes) showed 15.7% mean weight loss with tirzepatide 15 mg versus 3.3% with placebo at 72 weeks [8]. SURMOUNT-3 (N=579) followed a 12-week intensive lifestyle lead-in and then randomized participants to tirzepatide or placebo, showing an additional 18.4% loss from randomization baseline with tirzepatide 15 mg [9]. SURMOUNT-4 (N=670) demonstrated that stopping tirzepatide after 36 weeks of treatment led to regain of 14.8% body weight by week 88, while continuing the drug maintained 5.5% additional loss [10].

No direct head-to-head trial of semaglutide 2.4 mg versus tirzepatide in obesity has been published as of mid-2025. Indirect comparisons from network meta-analyses suggest tirzepatide 15 mg produces roughly 5 to 6 additional percentage points of weight loss compared with semaglutide 2.4 mg. That gap is clinically meaningful for patients seeking maximum weight reduction, though individual tolerability, cost, and insurance access often determine which drug a patient actually takes.

The AACE/ACE obesity clinical practice guidelines state that pharmacotherapy should be offered to patients with BMI <30 kg/m² and at least one weight-related comorbidity or BMI 30 or higher, and that drug selection should consider efficacy, safety profile, comorbidities, and patient preference [11].

Dosing, Titration, and the FDA-Approved Label

The Wegovy FDA label specifies a five-step titration schedule: semaglutide 0.25 mg weekly for weeks 1 to 4 to 0.5 mg for weeks 5 to 8 to 1.0 mg for weeks 9 to 12 to 1.7 mg for weeks 13 to 16, then the maintenance dose of 2.4 mg from week 17 onward [12]. That 16-week titration is designed to reduce GI intolerance, which is the primary driver of early discontinuation.

Patients who cannot tolerate the 2.4 mg maintenance dose after two consecutive weeks may remain at 1.7 mg for an additional four weeks before re-attempting escalation. If the 2.4 mg dose is still not tolerated, discontinuation should be considered given that the clinical trial data are for the 2.4 mg dose.

The Zepbound (tirzepatide) label follows a similarly gradual titration to a maximum dose of 15 mg weekly [13]. Both drugs are contraindicated in patients with a personal or family history of medullary thyroid carcinoma or multiple endocrine neoplasia syndrome type 2.

Who Qualifies and Who Should Not Take Semaglutide 2.4 mg

Based on the STEP trial inclusion criteria and the Wegovy FDA label, semaglutide 2.4 mg is indicated for adults with initial BMI <30 kg/m² or BMI <27 kg/m² with at least one weight-related comorbidity (hypertension, type 2 diabetes, dyslipidemia, obstructive sleep apnea, or cardiovascular disease) as an adjunct to a reduced-calorie diet and increased physical activity.

Contraindications include personal or family history of medullary thyroid carcinoma, multiple endocrine neoplasia syndrome type 2, known hypersensitivity to semaglutide or any excipient, and pregnancy. Pancreatitis history is not a strict contraindication in the label but warrants individualized benefit-risk assessment and close monitoring [12].

Patients with a prior history of gallbladder disease should be counseled that rapid weight loss of any cause increases cholelithiasis risk. STEP 1 reported cholelithiasis in 2.6% of semaglutide participants versus 1.2% of placebo participants, a difference that achieved statistical significance [1].

Monitoring Parameters During STEP-Protocol Dosing

Patients starting semaglutide 2.4 mg following the STEP titration protocol require periodic clinical assessment. Weight and BMI should be reassessed at 16 weeks: the Wegovy label advises discontinuation if a patient has not achieved at least 5% weight loss by that point, as they are unlikely to respond adequately [12].

Metabolic monitoring should include fasting glucose, HbA1c, and a lipid panel at baseline and again at six months. Renal function (eGFR) should be checked at baseline given semaglutide's modest effect on glomerular filtration in patients with pre-existing renal impairment. Blood pressure typically drops 3 to 5 mmHg within the first 12 weeks, which may require antihypertensive medication adjustment.