The one trial that put them in the same room
Almost every tirzepatide-versus-semaglutide comparison you will read is arithmetic between two separate trials with different patients, different lengths and different comparators. One trial actually randomized people between the two drugs. It ran 72 weeks, enrolled 751 adults with obesity and no type 2 diabetes, and reported a 6.5 percentage point difference in weight change. It was also open-label, which is the part the marketing leaves out.
What this comes down to
- SURMOUNT-5 reported a least-squares mean weight change at week 72 of -20.2% with tirzepatide and -13.7% with semaglutide, with the trial's stated P value below 0.001.
- Waist circumference moved -18.4 cm with tirzepatide and -13.0 cm with semaglutide over the same 72 weeks.
- The trial was phase 3b and open-label: everyone knew which drug they were taking. It compared maximum tolerated doses, not matched doses.
- It enrolled adults with obesity and without type 2 diabetes. It is not evidence about people with diabetes, and it measured weight and waist, not heart attacks.
What it found
SURMOUNT-5 randomly assigned adults with obesity and without type 2 diabetes, in a one-to-one ratio, to the maximum tolerated dose of tirzepatide or the maximum tolerated dose of semaglutide, once weekly for 72 weeks. Seven hundred and fifty-one people were randomized. The least-squares mean percent change in weight at week 72 was -20.2% with tirzepatide against -13.7% with semaglutide, and the least-squares mean change in waist circumference was -18.4 cm against -13.0 cm[1]. Participants on tirzepatide were more likely to reach reductions of at least 10%, 15%, 20% and 25%.
| What was measured | Tirzepatide | Semaglutide |
|---|---|---|
| Percent change in weight | -20.2% | -13.7% |
| Change in waist circumference | -18.4 cm | -13.0 cm |
| Dosing | 10 mg or 15 mg | 1.7 mg or 2.4 mg |
| Design | Open-label, phase 3b | Open-label, phase 3b |
SURMOUNT-5, primary and key secondary results at week 72
Three things it did not do
It was open-label. Every participant and every investigator knew which drug was in the pen, in a trial whose primary endpoint is a number on a scale and whose most common adverse events are symptoms people report. That is not a reason to discard the result; it is a reason to hold the size of it more loosely than a double-blind result of the same magnitude.
It compared maximum tolerated doses rather than matched ones, which is the comparison a patient actually faces and also the one that makes the two arms hardest to separate from each other on dose alone. And it enrolled people with obesity and without type 2 diabetes, so it says nothing about which drug to expect more from in diabetes, and nothing about cardiovascular events — the trial that measured those tested one drug against placebo, not against the other.
The most common adverse events in both arms were gastrointestinal, mostly mild to moderate, and concentrated during dose escalation. The trial reports that for both drugs, which is a useful corrective to the idea that one of the two is the gentle one. What escalation costs is a separate question, and it is mostly a money one: what the dose ladder charges works that out.
PubMed, via the E-utilities API
(tirzepatide[tiab] AND semaglutide[tiab]) AND "randomized controlled trial"[pt] AND "humans"[mh]
Returned 23. Positive control — semaglutide alone, identical trial filter — returned 316 through the identical filter in the same session.
Twenty-three records carry both drug names and the randomized-trial tag, and reading the top of that list by relevance shows what they mostly are: post hoc papers, quality-of-life substudies and indirect comparisons built on SURMOUNT-5 rather than second trials. One randomized head-to-head comparison generates a literature that looks larger than it is.
What it changes for somebody choosing
On the roster we read, both molecules are everywhere and the price gap between them is the thing that moves, not the evidence gap. 213 of the 257 sellers we have read price semaglutide in some form. The approved products themselves are not interchangeable either: only the semaglutide label carries a cardiovascular indication[4], and only the tirzepatide label carries a sleep apnea one[3]. Nor is the surrounding literature as large as it looks — twenty-three records carry both drug names and the randomized-trial tag, and most of them re-examine this one trial[2]. A trial result of this size is a reason to expect more from one drug on average; it is not a reason to accept a worse purchase. What the cash market actually charges and what a membership buys are the two pages that decide the rest of it.
Questions people actually ask
Is tirzepatide better than semaglutide?
In the one randomized head-to-head trial, tirzepatide produced a larger mean weight reduction over 72 weeks in adults with obesity and without type 2 diabetes. That is an average across 751 people in an open-label trial, not a prediction about one person, and it does not cover diabetes or cardiovascular outcomes.
How big was the difference in SURMOUNT-5?
The least-squares mean percent change in body weight at week 72 was -20.2% with tirzepatide and -13.7% with semaglutide, a difference of 6.5 percentage points, with the trial reporting a P value below 0.001.
Does open-label matter for a weight trial?
It matters more than for a hard endpoint like a heart attack. Knowing which drug you are on can influence what you eat, whether you stay in the trial, and which symptoms you report. It does not erase a 6.5 percentage point gap, but it is a reason to treat the exact figure as approximate.
Were the doses matched?
No. Both arms were titrated to a maximum tolerated dose: 10 mg or 15 mg of tirzepatide, and 1.7 mg or 2.4 mg of semaglutide. That is the comparison a patient faces in practice, but it means the trial compared two regimens rather than two equivalent doses.
Sources
Every source here was fetched and read for this article, with the identifier taken off the record that came back and the claim it supports written down beside it. All of it was read in September 2026, the same session the rest of this page draws on.
- 1.Tirzepatide as Compared with Semaglutide for the Treatment of Obesity. New England Journal of Medicine, 2025. PMID 40353578 · doi:10.1056/NEJMoa2416394A phase 3b, open-label, controlled trial randomized adults with obesity but without type 2 diabetes one-to-one to the maximum tolerated dose of tirzepatide (10 mg or 15 mg) or semaglutide (1.7 mg or 2.4 mg) once weekly for 72 weeks. A total of 751 participants underwent randomization. Least-squares mean percent change in weight at week 72 was -20.2% with tirzepatide and -13.7% with semaglutide; least-squares mean change in waist circumference was -18.4 cm and -13.0 cm respectively, both with P<0.001. The most common adverse events in both groups were gastrointestinal, mostly mild to moderate, occurring primarily during dose escalation.
- 2.PubMed, queried through the E-utilities API. National Library of Medicine, 2026. SourceOn 2026-09-12, records carrying both tirzepatide and semaglutide in the title or abstract with the randomized-trial publication type and human indexing returned 23, against 316 for semaglutide alone through the identical filter.
- 3.ZEPBOUND (tirzepatide) injection — FDA prescribing information. DailyMed, National Library of Medicine, Structured Product Labeling, 2026. Source · Document dated August 2026The approved tirzepatide weight product's indications, read from the SPL with effective time 20260828: reduction and long-term maintenance of excess body weight in adults with obesity or with overweight plus at least one weight-related comorbid condition, and treatment of moderate to severe obstructive sleep apnea in adults with obesity.
- 4.WEGOVY (semaglutide) injection and tablets — FDA prescribing information. DailyMed, National Library of Medicine, Structured Product Labeling, 2026. Source · Document dated June 2026The approved semaglutide weight product's indications, read from the SPL with effective time 20260618, include reduction of major adverse cardiovascular events in adults with established cardiovascular disease and either obesity or overweight, and reduction and long-term maintenance of excess body weight.
Key figures