Weight & Metabolic
Semaglutide vs Tirzepatide: What Do Head-to-Head Trials Show?
The short answer
One published head-to-head randomized trial exists: SURMOUNT-5, comparing the maximum tolerated doses of each drug in 751 adults with obesity. Tirzepatide produced significantly greater weight loss (-20.2% vs -13.7%) and a larger reduction in waist circumference (-18.4 cm vs -13.0 cm) over 72 weeks. That is real head-to-head evidence, not just a comparison across separate placebo trials — but it is also the only such trial, it excluded people with type 2 diabetes, and it did not test whether the larger weight loss translates into a larger cardiovascular or long-term health benefit.
Quick answers
- What is it?
- This covers the one published randomized trial (SURMOUNT-5) that directly compared semaglutide against tirzepatide in the same study, rather than comparing separate trials of each drug.
- Why are people interested in it?
- People choosing between the two most popular weight-loss drugs want to know which one actually works better, tested head-to-head rather than guessed at from separate trials.
- What has actually been studied?
- SURMOUNT-5, a randomized trial of 751 adults with obesity without diabetes, comparing the maximum tolerated doses of tirzepatide and semaglutide over 72 weeks, plus supporting data from the separate STEP 1 and SURMOUNT-1 placebo trials.
- How strong is the evidence?
- One real head-to-head randomized trial: tirzepatide produced significantly greater weight loss and waist-circumference reduction than semaglutide, but this is a single trial in one population that excluded people with type 2 diabetes.
- What don't we know yet?
- Whether the result holds in people with type 2 diabetes, whether more weight loss means more cardiovascular or mortality benefit, what an open-label design might have changed, and what happens beyond 72 weeks on head-to-head footing.
- Where can I read the original research?
- Sources at the bottom of this page: Aronne LJ, Horn DB, le Roux CW, et al., New England Journal of Medicine, 2025 (SURMOUNT-5); and Wilding JPH, Batterham RL, Calanna S, et al., New England Journal of Medicine, 2021 (STEP 1).
The short answer: Exactly one published randomized trial has put semaglutide and tirzepatide against each other directly: SURMOUNT-5, in 751 adults with obesity but without diabetes. Over 72 weeks, tirzepatide at its maximum tolerated dose produced significantly greater weight loss than semaglutide at its maximum tolerated dose (-20.2% vs -13.7%, P<0.001) and a larger drop in waist circumference (-18.4 cm vs -13.0 cm, P<0.001). That is genuine head-to-head evidence. But it is one trial, in one population, over one time window, and it did not test whether the bigger number on the scale produces a bigger benefit anywhere else.
What the research shows
The one trial that actually compared them
Before 2025, every claim that "tirzepatide works better than semaglutide" rested on cross-trial comparison — lining up separate placebo-controlled trials of each drug and comparing their results, a method vulnerable to differences in study populations, eras, and protocols. SURMOUNT-5, published in the New England Journal of Medicine in 2025, closed that gap. It randomized 751 adults with obesity (without type 2 diabetes) 1:1 to the maximum tolerated dose of tirzepatide (10 mg or 15 mg) or the maximum tolerated dose of semaglutide (1.7 mg or 2.4 mg), both once weekly for 72 weeks, in an open-label design.
The result: least-squares mean weight change was -20.2% with tirzepatide (95% CI, -21.4 to -19.1) versus -13.7% with semaglutide (95% CI, -14.9 to -12.6), P<0.001. Waist circumference fell 18.4 cm with tirzepatide versus 13.0 cm with semaglutide (P<0.001). Participants on tirzepatide were more likely to reach weight-loss thresholds of 10%, 15%, 20%, and 25% than those on semaglutide, though the trial's published abstract did not report the exact proportions at each threshold. The most common adverse events in both arms were gastrointestinal, mostly mild-to-moderate and concentrated during dose escalation — a pattern consistent with each drug's individual trial history.
What the earlier, indirect comparisons had already suggested
Before SURMOUNT-5, the cross-trial picture pointed the same direction. In STEP 1 (1,961 participants, 68 weeks), semaglutide 2.4 mg produced a mean weight change of -14.9% versus -2.4% with placebo. In SURMOUNT-1 (2,539 participants, 72 weeks), tirzepatide 15 mg produced a mean weight change of -20.9% versus -3.1% with placebo. Those numbers, from separate trials with separate placebo groups, lined up with what SURMOUNT-5 later confirmed directly — but a cross-trial comparison can't control for differences in enrolled populations or trial conduct the way a randomized head-to-head can. SURMOUNT-5 is the trial that removed that uncertainty for weight loss specifically.
What a single head-to-head trial can't tell you
A head-to-head trial answers one question well: which drug moved the number more, under these specific conditions. It does not automatically answer whether that translates into more health benefit. The two drugs' major outcome trials are not comparable in design. Semaglutide's cardiovascular trial, SELECT, was placebo-controlled in people with obesity and existing cardiovascular disease. Tirzepatide's ongoing cardiovascular trial, SURPASS-CVOT, is not placebo-controlled at all — it compares tirzepatide against dulaglutide, an active GLP-1 drug with its own established cardiovascular benefit, in 13,299 people with type 2 diabetes and atherosclerotic disease. As of its design publication, SURPASS-CVOT was fully recruited but still ongoing, with no results reported. There is no trial, published or in progress, that puts semaglutide and tirzepatide head-to-head on cardiovascular outcomes.
There's also a downstream question SURMOUNT-5 wasn't built to answer: what happens after people stop. A follow-on trial randomized a subset of SURMOUNT-5 participants who had completed either arm (205 previously on tirzepatide, 171 previously on semaglutide) to a third drug, oral orforglipron, or placebo, to see how much of the lost weight was maintained. Both placebo subgroups regained substantial weight after their originating drug was withdrawn — this speaks to durability after stopping, not to which of the two original drugs was "better," but it's a reminder that a single 72-week snapshot doesn't capture what happens next.
What we don't know yet
Whether SURMOUNT-5's result holds in people with type 2 diabetes. The trial explicitly excluded them. Cross-trial data in diabetes populations exist for each drug separately, but no head-to-head trial has been conducted in people with diabetes.
Whether more weight loss means more cardiovascular or mortality benefit. No trial has tested semaglutide against tirzepatide on cardiovascular events, kidney outcomes, or mortality. SELECT and SURPASS-CVOT are different trials, in different populations, with different comparators — they cannot be stacked against each other to answer this.
What blinding would have changed. SURMOUNT-5 was open-label: participants and investigators knew which drug was assigned. For an objective endpoint like body weight this matters less, but it could plausibly affect adherence, reported side effects, or dropout behavior in ways a blinded trial would control for.
What happens beyond 72 weeks on head-to-head footing. The only direct comparison trial ran for 72 weeks. Whether the gap between the drugs widens, narrows, or holds steady over two or three years of continued treatment has not been tested in a head-to-head design.
Whether the gap is dose-dependent in a way that matters clinically. Both drugs were compared at their maximum tolerated doses. Whether the same 6-7 percentage point gap in weight loss holds at lower, more commonly tolerated doses of each drug was not addressed by this trial.
Sources
- Aronne LJ, Horn DB, le Roux CW, Ho W, Falcon BL, Gomez Valderas E, Das S, Lee CJ, Glass LC, Senyucel C, Dunn JP. Tirzepatide as Compared with Semaglutide for the Treatment of Obesity. New England Journal of Medicine, 2025;393(1):26-36. DOI
- Wilding JPH, Batterham RL, Calanna S, et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity. New England Journal of Medicine, 2021;384(11):989-1002. DOI
- Jastreboff AM, Aronne LJ, Ahmad NN, et al. Tirzepatide Once Weekly for the Treatment of Obesity. New England Journal of Medicine, 2022;387(3):205-216. DOI
- Nicholls SJ, Bhatt DL, Buse JB, et al. Comparison of tirzepatide and dulaglutide on major adverse cardiovascular events in participants with type 2 diabetes and atherosclerotic cardiovascular disease: SURPASS-CVOT design and baseline characteristics. American Heart Journal, 2023;267:1-11. DOI
- Aronne LJ, Horn DB, le Roux CW, et al. Orforglipron for maintenance of body weight reduction: the double-blind, randomized phase 3b ATTAIN-MAINTAIN trial. Nature Medicine, 2026;32(7):2679-2687. DOI
Article records maintained via PubMed. Peptide Proof reports what studies found. It does not provide medical advice, dosing guidance, or treatment recommendations.
Topic: Weight & Metabolism | Evidence Index: semaglutide · tirzepatide
Sources
- Tirzepatide as Compared with Semaglutide for the Treatment of Obesity (SURMOUNT-5)
- New England Journal of Medicine, 2025 · n=751 · DOI
- Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1)
- New England Journal of Medicine, 2021 · n=1961 · DOI
- Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1)
- New England Journal of Medicine, 2022 · n=2539 · DOI
- Comparison of tirzepatide and dulaglutide on major adverse cardiovascular events (SURPASS-CVOT design)
- American Heart Journal, 2023 · n=13299 · DOI
- Orforglipron for maintenance of body weight reduction (ATTAIN-MAINTAIN, post-SURMOUNT-5 cohort)
- Nature Medicine, 2026 · n=376 · DOI
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