SURMOUNT-1 Trial: Tirzepatide for Obesity

Trial design

SURMOUNT-1 (NCT04184622) was a 72-week, randomized, double-blind, placebo-controlled, multicenter Phase 3 trial. Investigators enrolled 2,539 adults aged 18 years or older with a body mass index of 30 kg/m² or greater, or 27 kg/m² or greater with at least one weight-related comorbidity (hypertension, dyslipidemia, obstructive sleep apnea, or cardiovascular disease). Participants with type 2 diabetes were excluded.

Randomization was 1:1:1:1 to tirzepatide 5 mg, 10 mg, or 15 mg weekly, or placebo, all combined with lifestyle intervention. Tirzepatide doses were titrated upward by 2.5 mg every 4 weeks until the assigned maintenance dose was reached, then maintained for the remainder of the 72-week treatment period.

Primary outcomes — dose response

At week 72, mean change in body weight was −15.0% with tirzepatide 5 mg, −19.5% with 10 mg, and −20.9% with 15 mg, versus −3.1% with placebo (all comparisons P<0.001)¹. Mean absolute weight loss in the 15 mg group was approximately 23.6 kg (52 lb).

The proportion of participants achieving at least 5% weight loss was 85.1%, 88.9%, and 90.9% across the three doses versus 34.5% with placebo. At least 15% weight loss was achieved by 50.1%, 65.3%, and 70.6% of tirzepatide-treated participants. At least 20% weight loss — a threshold previously achieved primarily through bariatric surgery — was met by 30.0%, 50.1%, and 56.7% across the three doses, versus 3.1% with placebo.

Secondary outcomes

Cardiometabolic improvements paralleled weight loss. Mean waist circumference decreased by 14.0, 17.7, and 18.5 cm across the three doses versus 4.0 cm with placebo. Systolic blood pressure decreased by 6.2, 7.7, and 8.0 mm Hg versus 1.0 mm Hg with placebo. Fasting plasma glucose, fasting insulin, and HbA1c all decreased significantly in tirzepatide-treated groups despite the absence of diabetes at baseline.

Lipid changes included reductions in triglycerides (24%, 25%, and 25% across doses vs 6% placebo) and increases in HDL cholesterol (3.4%, 4.6%, and 4.8% vs 0.4% placebo). Quality-of-life measures including IWQOL-Lite-CT, SF-36, and EQ-5D-5L improved significantly with all tirzepatide doses.

Adverse events and tolerability

Gastrointestinal events were the most common adverse events and the most common reason for discontinuation. Nausea (28% to 33% across doses vs 9.5% placebo), diarrhea (19% to 23% vs 7.3% placebo), constipation (17% to 12% vs 6.3% placebo), and vomiting (8% to 13% vs 1.6% placebo) were most prominent during dose titration. The proportion of participants discontinuing due to adverse events ranged from 4.3% to 7.1% across tirzepatide doses versus 2.6% with placebo¹.

Serious adverse events were similar across groups (5.5% to 6.9% with tirzepatide vs 6.0% with placebo). Pancreatitis occurred in less than 0.5% of any group and was not statistically elevated. Gallbladder-related events were modestly more common with tirzepatide, consistent with rapid weight loss as a known risk factor.

How SURMOUNT-1 compares with STEP-1

SURMOUNT-1 (tirzepatide 15 mg, 72 weeks, 20.9% weight loss) and STEP-1 (semaglutide 2.4 mg, 68 weeks, 14.9% weight loss) were not direct head-to-head trials but used similar designs. The 6-percentage-point greater weight loss with tirzepatide 15 mg is hypothesized to reflect the dual mechanism: tirzepatide activates both the GIP and GLP-1 receptors, while semaglutide activates only GLP-1.

The SURPASS-2 trial provided a direct head-to-head comparison in adults with type 2 diabetes. At 40 weeks, tirzepatide 5/10/15 mg produced 7.6%, 9.3%, and 11.2% mean weight loss versus 5.7% with semaglutide 1 mg (the maximum dose approved at the time)². Direct comparisons at the higher Wegovy-equivalent semaglutide dose (2.4 mg) are not yet available in randomized form.

Implications and ongoing program

Subsequent SURMOUNT trials extended the evidence base. SURMOUNT-2 evaluated tirzepatide in adults with obesity and type 2 diabetes (15.7% mean weight loss at 72 weeks)³. SURMOUNT-3 used a 12-week intensive lifestyle intervention lead-in and showed an additional 18.4% weight loss with tirzepatide added on. SURMOUNT-4 evaluated maintenance after 36 weeks of run-in and demonstrated continued weight loss with tirzepatide versus regain with placebo, consistent with the STEP-4 finding for semaglutide⁴.

The SURMOUNT-OSA trial demonstrated significant improvement in obstructive sleep apnea severity with tirzepatide⁵. Ongoing trials are evaluating cardiovascular outcomes (SURMOUNT-MMO), heart failure (SUMMIT), and other obesity-related comorbidities.

Related editorial coverage

• Tirzepatide profile: mechanism, dosing, and FDA status
• STEP-1 trial: semaglutide for obesity
• Dual GIP/GLP-1 agonism — pharmacology
• Weight-loss therapeutics category

Frequently asked questions

How much weight did people lose on tirzepatide in SURMOUNT-1?

Mean weight loss at 72 weeks was 15.0% with tirzepatide 5 mg, 19.5% with 10 mg, and 20.9% with 15 mg, versus 3.1% with placebo. More than half of participants on tirzepatide 15 mg achieved at least 20% body weight loss¹.

How does tirzepatide compare with semaglutide in head-to-head trials?

SURPASS-2, in adults with type 2 diabetes, showed tirzepatide 5/10/15 mg produced 7.6%/9.3%/11.2% weight loss versus 5.7% with semaglutide 1 mg over 40 weeks². Indirect comparison with semaglutide 2.4 mg from STEP-1 suggests tirzepatide 15 mg produces approximately 6 percentage points greater weight loss.

What is the maintenance dose of tirzepatide for obesity?

FDA-approved maintenance doses are 5, 10, and 15 mg weekly. Higher doses produce greater weight loss but more gastrointestinal side effects. The optimal dose for a given patient is determined by clinical response and tolerability during dose titration.

Was tirzepatide effective in patients with diabetes?

SURMOUNT-1 excluded patients with type 2 diabetes. SURMOUNT-2 evaluated tirzepatide in adults with obesity and type 2 diabetes and showed 15.7% mean weight loss at 72 weeks — slightly less than in nondiabetic populations but still substantially greater than any prior pharmacotherapy in this group³.