Tirzepatide vs Semaglutide: What the Head-to-Head Trial Found

Until 2025 the comparison between these two drugs was guesswork. People lined up results from separate trials, run in different populations, and drew conclusions the data could not support. Then someone actually randomised people to one or the other and followed them for 72 weeks.

How were tirzepatide and semaglutide compared?

SURMOUNT-5 ran from April 2023 to November 2024 across 32 sites in the United States and Puerto Rico. It enrolled 751 adults, mean age 45, with a BMI of 30 or above, or 27 and above with a weight-related complication, and, importantly, without diabetes.

Both arms were titrated to the maximum tolerated dose: 10 or 15 mg weekly for tirzepatide, 1.7 or 2.4 mg weekly for semaglutide. Everyone was followed for 72 weeks.

That design matters. This is not a comparison of one trial’s participants against another’s. It is the same population, the same sites, the same 72 weeks, differing only in which drug they were assigned.

Which is better for weight loss, tirzepatide or semaglutide?

The weight difference was statistically significant at p<0.001, as was the waist measurement.

The waist number is worth dwelling on, because it is the one most relevant to health rather than to a scale reading. Visceral fat (the fat around the organs, which is what waist circumference approximates) is the fraction most strongly tied to cardiovascular and metabolic risk. An 18 cm reduction is not a cosmetic result.

Why do men lose less weight on these drugs?

Buried in the trial’s results is a line that changes how a man should read every other number on this page: weight loss was approximately 6% lower in men than in women, in both treatment arms.

That is not a small adjustment. The headline figures, 20.2% and 13.7%, are averages across a trial population that was, like most obesity trials, majority female. A man reading those numbers as a personal forecast is reading a number generated mostly by women.

It does not mean the drugs do not work in men. It means the honest expectation for a man is somewhat below the advertised average, and that being disappointed by a result that is entirely normal for his sex is a real risk.

It also makes the things that are within your control matter more, not less, particularly resistance training and protein intake, which are what stand between you and losing a larger share of that weight as muscle rather than fat.

Why does tirzepatide work better?

The two drugs are not the same class of molecule, despite being discussed as though they were.

Semaglutide is a GLP-1 receptor agonist: it mimics one gut hormone. Tirzepatide is a dual agonist, acting at both the GLP-1 receptor and the GIP receptor: two hormones rather than one.

Whether the second target fully explains the difference is not settled. But it is the most plausible mechanistic account of why a directly randomised comparison produced a gap this size, and it is the reason the two should not be treated as interchangeable when a pharmacy offers a substitution.

Which has worse side effects?

The usual trade-off in drug comparisons is that the more effective option is harder to tolerate. That is not what happened here.

Gastrointestinal side effects severe enough to stop treatment occurred in 2.7% of the tirzepatide group against 5.6% of the semaglutide group, roughly half the rate, in the arm that also lost more weight. Most adverse events in both arms were mild to moderate and concentrated during dose escalation, which is the usual pattern and is covered in more detail in the side-effects article.

Which one should you actually take?

On trial data alone, tirzepatide won on efficacy, on waist reduction and on tolerability. In practice, four things usually decide it before the trial data get a look in.

Coverage. What your insurer will pay for is, for most people, the actual determinant. Formularies differ and change.

Supply. Both drugs have had shortages. The one you can reliably obtain beats the one you cannot.

Your other conditions. Semaglutide now carries indications beyond weight: cardiovascular risk reduction in people with established disease, and liver disease in a specific population. If one of those applies to you, the calculus changes.

What you tolerate. Population averages do not predict individuals. Some people are fine on one and miserable on the other.

One thing that should not decide it is price on a website that does not require a prescription. What you are actually buying there is a different product from the one in this trial.

And whichever you take, the number that matters most is not in this article. It is what happens when you stop.

This article is for information only and is not medical advice. It cannot account for your individual circumstances. Talk to a doctor about your own situation, particularly before starting or stopping any prescription medication.