Tirzepatide Weight Loss: What Clinical Trials Report

Two landmark clinical trials provide the weight-loss data for tirzepatide. SURMOUNT-1 (2022) tested it against placebo in 2,539 adults. SURMOUNT-5 (2025) tested it head-to-head against semaglutide in 751 adults. Both were published in the New England Journal of Medicine with verified PubMed records. This page presents the numbers directly from those trials.

SURMOUNT-1 Trial Results

The SURMOUNT-1 trial (Jastreboff et al., NEJM 2022, PMID 35658024) enrolled 2,539 adults with obesity (BMI 30 or above, or 27 with a comorbidity) who did not have type 2 diabetes. Participants were randomized to tirzepatide 5 mg, 10 mg, 15 mg or placebo, administered once weekly for 72 weeks.

Study Design and Population

Participants had an average BMI of approximately 38 and an average body weight of approximately 105 kg at baseline. The study excluded people with type 2 diabetes. Lifestyle counseling (reduced-calorie diet and increased physical activity) was provided to all groups including placebo. The trial was double-blind and placebo-controlled.

Weight Loss by Dose

Weight loss was dose-dependent. At 5 mg, average loss was 15.0%. At 10 mg, 19.5%. At 15 mg, 20.9%. The placebo group lost 3.1%, consistent with lifestyle intervention alone. More than half of participants on the 15 mg dose achieved 20% or greater weight loss, a threshold rarely seen with previous pharmacotherapies.

Adverse Events Reported

The most common adverse events were gastrointestinal: nausea, diarrhea and constipation. These occurred primarily during dose escalation and were mostly mild to moderate. Discontinuation rates due to adverse events ranged from 2.6% (placebo) to 7.1% (10 mg group). Serious adverse events were not significantly different between treatment and placebo groups.

SURMOUNT-5 Trial: Tirzepatide vs Semaglutide

The SURMOUNT-5 trial (Aronne et al., NEJM 2025, PMID 40353578) was the first head-to-head comparison of tirzepatide and semaglutide for weight loss. It enrolled 751 adults with obesity without diabetes and randomized them to tirzepatide or semaglutide for 72 weeks.

Head-to-Head Comparison Design

This was an open-label, randomized trial comparing maximum tolerated doses of each drug over 72 weeks. Tirzepatide was dosed up to 15 mg weekly; semaglutide up to 2.4 mg weekly. Both groups received lifestyle counseling. The primary endpoint was percent change in body weight.

Weight Loss Outcomes

Tirzepatide produced 20.2% average weight loss versus 13.7% for semaglutide, a 6.5 percentage-point difference that was statistically significant (p less than 0.001). Tirzepatide participants also more frequently achieved categorical weight loss thresholds of 10%, 15%, 20% and 25%.

Safety Comparison

Both drugs had similar adverse event profiles dominated by gastrointestinal symptoms (nausea, diarrhea, constipation), predominantly mild to moderate and concentrated during dose escalation. The trial found that fewer participants stopped tirzepatide due to stomach-related side effects compared to semaglutide, though tirzepatide caused more injection-site reactions.

What the Numbers Mean for Buyers

Clinical trial data provides the evidence basis for tirzepatide's weight-loss claims. But there are important caveats for anyone buying tirzepatide, especially compounded formulations.

These Were Research Subjects, Not Customers

Trial participants were screened, monitored and supported by research teams. They received standardized doses of FDA-approved tirzepatide alongside structured lifestyle counseling. Real-world results in a telehealth setting with compounded medication may differ. The controlled conditions of a clinical trial do not perfectly predict individual outcomes.

Individual Results Vary

The 15-21% figures are averages across hundreds or thousands of participants. Some individuals lost more; some lost less. Factors including baseline weight, adherence to the injection schedule, diet, physical activity and individual metabolic response all influence outcomes. Your provider can discuss realistic expectations during the consultation.

Compounded Tirzepatide Was Not Studied

Both SURMOUNT-1 and SURMOUNT-5 used FDA-approved tirzepatide manufactured by Eli Lilly. Compounded tirzepatide has not been tested in controlled clinical trials. The active ingredient is the same, but the manufacturing process, potency verification and quality controls differ between FDA-approved and compounded products. We cannot assume trial results apply identically to compounded formulations.