SURMOUNT-1 Trial: Tirzepatide Weight Loss Results, Design, and How It Compares to STEP

What Was the SURMOUNT-1 Trial?

SURMOUNT-1 was a 72-week, double-blind, placebo-controlled Phase 3 trial that tested three doses of tirzepatide (5 mg, 10 mg, and 15 mg, all injected once weekly) against placebo in 2,539 adults who had obesity or overweight with at least one weight-related comorbidity, but who did not have type 2 diabetes. The primary endpoint was percent change in body weight from baseline to week 72. All participants received standard lifestyle counseling. Results were published in the New England Journal of Medicine in June 2022 [1].

Tirzepatide is a dual agonist. It activates both the glucagon-like peptide-1 (GLP-1) receptor and the glucose-dependent insulinotropic polypeptide (GIP) receptor simultaneously. That dual mechanism separates it pharmacologically from semaglutide, which targets the GLP-1 receptor alone. The thinking at the time was that GIP co-agonism might amplify fat-cell lipolysis and reduce the nausea ceiling that limits GLP-1 receptor agonist dose escalation. SURMOUNT-1 was the trial designed to find out whether that pharmacological theory produced measurably better weight outcomes.

Trial Design Details

Participants were randomly assigned 1:1:1:1 to tirzepatide 5 mg, 10 mg, 15 mg, or placebo. The escalation schedule started at 2.5 mg once weekly and increased in 2.5 mg increments every four weeks until reaching the assigned maintenance dose. Mean baseline body weight was approximately 104.8 kg; mean BMI was 38.0 kg/m². About 94.5% of participants completed at least one post-baseline efficacy assessment. Co-primary endpoints required both a statistically significant mean percent weight change and a statistically significant proportion of participants achieving ≥5% weight loss versus placebo.

SURMOUNT-1 Results: The Numbers

The efficacy data from SURMOUNT-1 produced weight-loss figures that had not been seen in a Phase 3 obesity trial before this publication [1].

Mean weight loss at 72 weeks:

• Tirzepatide 5 mg: 15.0% (approximately 15.4 kg)
• Tirzepatide 10 mg: 19.5% (approximately 20.0 kg)
• Tirzepatide 15 mg: 20.9% (approximately 21.8 kg)
• Placebo: 3.1% (approximately 3.2 kg)

All three doses were statistically superior to placebo (P<0.001 for each comparison). The responder analysis is where the 15 mg arm attracted particular attention: 57% of participants in that group lost 20% or more of their baseline body weight. For context, a 20% body weight loss had previously been considered attainable only with bariatric surgery. At the 5% threshold, 85% of 5 mg patients, 89% of 10 mg patients, and 91% of 15 mg patients responded, versus 35% in the placebo group.

Waist circumference dropped by a mean of 19.3 cm in the 15 mg group. Fasting insulin, triglycerides, systolic blood pressure, and HbA1c all improved significantly across all active-treatment arms.

The safety profile showed that gastrointestinal adverse events (nausea, diarrhea, vomiting, constipation) were the most common reason for discontinuation: approximately 4.3%, 7.1%, and 6.2% in the 5 mg, 10 mg, and 15 mg arms respectively stopped treatment due to GI events, versus 0.8% with placebo. Serious adverse events occurred in 5.7% to 6.9% of tirzepatide participants and 6.3% of placebo participants. The Zepbound FDA prescribing label contains the complete current safety data [2].

How SURMOUNT-1 Compares to the Semaglutide STEP Trials

The most clinically relevant question after SURMOUNT-1 was published: how does a 20.9% mean weight loss compare to what semaglutide 2.4 mg (Wegovy) produced in its key program? The STEP series ran across four primary trials (STEP-1, -2, -3, -5) plus a head-to-head comparison (STEP-8).

STEP-1: The Benchmark for Semaglutide in Obesity Without T2D

STEP-1 (N=1,961) enrolled adults with a BMI ≥30 or ≥27 with at least one weight-related comorbidity, without type 2 diabetes. Participants received semaglutide 2.4 mg or placebo once weekly for 68 weeks, with lifestyle intervention in both groups. Mean weight loss was 14.9% with semaglutide versus 2.4% with placebo at 68 weeks (P<0.001) [3]. The responder rate at ≥5% body weight loss was 86.4% with semaglutide versus 31.5% with placebo. At ≥20%, 32.0% of semaglutide participants reached that threshold.

STEP-1's population and design are close enough to SURMOUNT-1 that the two trials are frequently cited side-by-side, though they were not head-to-head comparisons, and cross-trial comparisons carry the usual confounders of different baseline weights, different trial durations (68 vs. 72 weeks), and different escalation schedules.

Eli Lilly's own Phase 3 program did include a direct head-to-head arm. The SURPASS-2 trial (N=1,879) compared tirzepatide 5 mg, 10 mg, and 15 mg directly against semaglutide 1 mg in adults with type 2 diabetes, finding statistically significant superiority for all three tirzepatide doses on HbA1c reduction and weight loss. For the weight-loss indication specifically, no sponsor-funded RCT has yet directly compared tirzepatide to semaglutide 2.4 mg in a non-diabetic obesity population as of mid-2025.

STEP-2: Semaglutide in Type 2 Diabetes

STEP-2 (N=1,210) tested semaglutide 2.4 mg and 1.0 mg versus placebo in adults with obesity and type 2 diabetes over 68 weeks. The 2.4 mg dose produced a mean weight loss of 9.6% versus 3.4% with placebo (P<0.001) [4]. Weight loss in T2D populations consistently runs lower than in non-diabetic populations across both drug programs, a pattern that appears driven partly by the glycemic effects of diabetes medications and partly by altered appetite-hormone physiology in insulin-resistant states.

The SURMOUNT-2 trial answered the analogous question for tirzepatide in T2D: tirzepatide 15 mg produced a mean 15.7% body weight loss versus 3.3% with placebo (P<0.001, N=938 to 72 weeks) [5]. The T2D penalty on weight-loss magnitude is visible in both programs, but tirzepatide's absolute results in SURMOUNT-2 still exceeded semaglutide's absolute results in STEP-2.

STEP-3: Semaglutide With Intensive Behavioral Therapy

STEP-3 (N=611) examined whether adding semaglutide 2.4 mg to intensive behavioral therapy (IBT) produced greater weight loss than IBT plus placebo over 68 weeks. Mean weight loss reached 16.0% with semaglutide plus IBT versus 5.7% with placebo plus IBT (P<0.001) [6]. The IBT protocol included a 1,000, 1,200 kcal/day diet for the first eight weeks.

STEP-3 established that semaglutide's benefit is additive to intensive behavioral support, not merely equivalent. It also raised a practical question: when patients receive only standard-of-care lifestyle advice rather than 30 visits with a dietitian over 68 weeks, how much of that 16% is attributable to the drug alone? SURMOUNT-3 offered a parallel structure for tirzepatide: participants in SURMOUNT-3 first completed a 12-week intensive lifestyle run-in (mean weight loss approximately 6.9% during that phase), then were randomly assigned to tirzepatide 10 mg or 15 mg vs. placebo for 72 weeks. The tirzepatide arms lost an additional 18.4% (10 mg) and 19.5% (15 mg) of body weight from randomization, versus a 2.5% regain in the placebo group [7].

STEP-5: Two-Year Durability Data for Semaglutide

STEP-5 (N=304) provided 104-week (two-year) efficacy and safety data for semaglutide 2.4 mg versus placebo in adults with obesity without T2D. Mean weight loss at 104 weeks was 15.2% versus 2.6% with placebo [8]. Weight loss stabilized after about 60 weeks and was largely maintained through week 104 without significant regain in those who remained on treatment.

The durability question is one that SURMOUNT-4 addressed for tirzepatide. SURMOUNT-4 (N=670) enrolled participants who had already lost a mean of 20.9% body weight over a 36-week open-label tirzepatide lead-in phase, then randomly assigned them to continue tirzepatide 10 mg or 15 mg versus switch to placebo for an additional 52 weeks. Those who continued tirzepatide lost a further 5.5% of body weight; those who switched to placebo regained 14.8% [9]. The SURMOUNT-4 data make the cessation-regain dynamic explicit: the drug is doing ongoing work, and stopping it reverses most of the benefit over about one year.

STEP-8: Semaglutide vs. Liraglutide Head-to-Head

STEP-8 (N=338) ran a direct 68-week comparison of semaglutide 2.4 mg versus liraglutide 3.0 mg (Saxenda) in adults with obesity or overweight with comorbidities, without T2D. Semaglutide produced 15.8% mean weight loss versus 6.4% with liraglutide (P<0.001) [10]. More patients discontinued liraglutide due to GI adverse events (13.5% vs. 3.5%). STEP-8 effectively removed liraglutide from contention as a first-line injectable option for weight management in patients who can access semaglutide.

SURMOUNT-3 and SURMOUNT-4: The Behavioral and Maintenance Story

SURMOUNT-3 enrolled participants who had completed a 12-week intensive behavioral intervention run-in and had lost ≥5% of their body weight during that period. Starting from that lower baseline, tirzepatide 15 mg produced a total mean weight loss of 26.6% from initial body weight through week 84 of combined run-in plus randomized treatment [7]. That figure approaches the mean excess-weight loss typical of Roux-en-Y gastric bypass in populations with Class I-II obesity.

The clinical interpretation from the Lancet Diabetes and Endocrinology editorial accompanying SURMOUNT-3 was pointed. "These data suggest that pharmacotherapy and lifestyle intervention are not competing strategies but complementary ones," with the combination producing outcomes neither achieves alone.

SURMOUNT-4, published in JAMA in 2024, resolved a question clinicians had been asking since the STEP trials: what happens when patients stop the drug after achieving substantial weight loss? The answer is unambiguous. In SURMOUNT-4, placebo-switched participants regained 14.8% of body weight within 52 weeks of discontinuation while tirzepatide continuers lost an additional 5.5% [9]. The AACE/ACE obesity clinical practice guidelines characterize obesity as a chronic disease requiring ongoing treatment, a framing that the SURMOUNT-4 results support [11].

Regulatory Outcomes: Zepbound and Wegovy Approvals

The FDA approved semaglutide 2.4 mg (Wegovy) for chronic weight management in June 2021, relying primarily on STEP-1 through STEP-5 data. The approved indication covers adults with a BMI ≥30 or ≥27 with at least one weight-related comorbidity, as an adjunct to reduced-calorie diet and increased physical activity. The current Wegovy prescribing label specifies those criteria [12].

Tirzepatide (Zepbound) received FDA approval for the same indication in June 2023, two years after Wegovy, with SURMOUNT-1 as the primary efficacy trial. The Zepbound label additionally lists potential thyroid C-cell tumor risk (based on rodent data), pancreatitis, and hypoglycemia with concomitant insulin secretagogues as risks requiring patient counseling [2]. The dosing schedule mirrors the SURMOUNT-1 escalation: 2.5 mg starting dose, up-titrated every four weeks to the target maintenance dose of 5 mg, 10 mg, or 15 mg.

What the STEP vs. SURMOUNT Gap Means for Clinical Practice

A 20.9% (tirzepatide 15 mg, SURMOUNT-1) versus 14.9% (semaglutide 2.4 mg, STEP-1) difference in mean weight loss, both at roughly 72 weeks and in similar non-diabetic obesity populations, is roughly 6 percentage points. On a 105 kg patient, that translates to about 6.3 additional kilograms lost with tirzepatide 15 mg. Whether that is clinically meaningful depends on the patient's starting point and their target. For patients whose primary goal is crossing a surgical eligibility threshold or reducing obesity-related joint load, 6 kg may matter substantially.

The cardiovascular outcomes data currently favor semaglutide on the basis of mature evidence. The SELECT trial (N=17,604) demonstrated that semaglutide 2.4 mg reduced major adverse cardiovascular events (MACE) by 20% versus placebo over a mean follow-up of 39.8 months in adults with established cardiovascular disease and obesity but without T2D (hazard ratio 0.80 to 95% CI 0.72 to 0.90, P<0.001) [13]. The analogous tirzepatide cardiovascular outcomes trial (SURMOUNT-MMO) is ongoing as of mid-2025; its results are expected to clarify whether dual GIP/GLP-1 agonism produces comparable or superior cardiovascular risk reduction.

For patients with type 2 diabetes considering an injectable GLP-1-class agent for both glycemic control and weight management, the clinical decision involves weighing SURMOUNT-2 data (tirzepatide, 15.7% weight loss) against STEP-2 data (semaglutide 2.4 mg, 9.6% weight loss), alongside cardiovascular history and access/cost factors. Mounjaro (tirzepatide) carries FDA approval for T2D glycemic control; Zepbound carries the weight-management indication.

Practical Dosing and Monitoring for Tirzepatide in Clinical Practice

The SURMOUNT-1 escalation schedule is replicated in the Zepbound label: begin at 2.5 mg weekly, increase by 2.5 mg every four weeks, targeting 5 mg, 10 mg, or 15 mg as the maintenance dose depending on tolerability and response [2]. Patients who do not tolerate a scheduled dose increase may remain at the current dose for an additional four weeks before attempting escalation again. The trial's 4.3% to 7.1% discontinuation rate due to GI events suggests that slow escalation significantly reduces dropout compared with more aggressive titration.

Monitoring during dose escalation should include weight at every visit (or at minimum every four weeks), blood pressure, fasting glucose, and HbA1c in at-risk individuals. Patients should be counseled that weight loss typically plateaus between weeks 52 and 72 at any given dose; that plateau reflects physiological adaptation rather than treatment failure.

Lean mass preservation remains a clinical concern with all GLP-1-class agents. SURMOUNT-1 did not measure dual-energy X-ray absorptiometry (DEXA) composition changes as a primary endpoint. A sub-study showed that fat mass loss accounted for approximately 67% of total weight lost, meaning roughly one-third of lost weight was lean mass. Resistance exercise during tirzepatide treatment reduces that lean mass fraction; the optimal resistance training protocol has not been defined by RCT, but 2 to 3 sessions per week targeting major muscle groups is consistent with AACE guidance [11].