Tirzepatide vs Semaglutide: SURMOUNT-5 Head-to-Head
Two of the most-discussed weight-loss molecules were finally put in the same trial — here is who won, by how much, and what the win does not tell you.
Abstract: SURMOUNT-5 is the first large head-to-head obesity trial of tirzepatide versus semaglutide, and it gave tirzepatide a clear win — 20.2% versus 13.7% mean weight loss at 72 weeks. This guide breaks down the design, the responder numbers, the GIP-receptor mechanism behind the gap, and the four questions the trial deliberately leaves open.
For two years, every tirzepatide vs semaglutide debate ran on cross-trial math: tirzepatide’s SURMOUNT-1 number lined up next to semaglutide’s STEP-1 number, with an asterisk warning that the two came from different trials in different people. SURMOUNT-5 removed the asterisk. It randomized 751 adults with obesity to one drug or the other, ran both to their maximum obesity doses for 72 weeks, and measured them the same way at the same time. The result is the single most decision-relevant comparison in the class — and, like every head-to-head, it answers one question cleanly while leaving several others wide open.
This is an educational, research-focused breakdown, not medical advice. Tirzepatide, semaglutide and retatrutide are sold on this site for laboratory research use only; the branded medicines (Zepbound, Wegovy) are prescription pharmaceuticals referenced here only to describe the published evidence.
Tirzepatide vs semaglutide: why only a head-to-head counts
Almost every ranking of weight-loss drugs you will read — including parts of our own GLP-1 weight-loss class ranking — is built by placing the headline number from one trial next to the headline number from another. That is useful for a rough order, but it is not evidence that one drug beats another, for three reasons:
- Different populations lose different amounts. A drug tested in people without diabetes will post a bigger number than the same drug tested in people with diabetes, who consistently lose less.
- Different statistical estimands. Trials can report the effect in everyone randomized (the conservative “treatment-regimen” number) or only in people who stayed on the drug (the flattering “efficacy” number). Quote one drug’s flattering estimand against another’s conservative one and the ranking can flip.
- Different trial conditions. Site, era, diet-and-exercise advice, titration schedule, and dropout handling all move the number independently of the molecule.
A head-to-head trial cancels all three at once: same patients, same estimand, same conditions, two drugs. Whatever gap remains is attributable to the drugs. That is why SURMOUNT-5 carries more weight for the tirzepatide vs semaglutide question than the dozen separate trials that preceded it combined.
What SURMOUNT-5 actually measured
SURMOUNT-5 was a Phase 3b, randomized, open-label trial that ran the two drugs at their real obesity doses against each other.
- Who: 751 adults (mean age ~45) with obesity, or overweight plus at least one weight-related complication, and crucially without type 2 diabetes — the cleaner-responding population that also makes the comparison fairer, since neither drug was handicapped by diabetes.
- What: maximum tolerated tirzepatide (10 mg or 15 mg) versus maximum tolerated semaglutide (1.7 mg or 2.4 mg), each given subcutaneously once weekly, randomized 1:1.
- How long: 72 weeks.
- Primary endpoint: percent change in body weight at week 72.
The dose detail matters. An earlier head-to-head, the SURPASS-2 diabetes trial, also favored tirzepatide — but it used semaglutide 1 mg, a diabetes dose, not the higher obesity dose, so semaglutide arguably fought with one hand tied. SURMOUNT-5 fixed that by running semaglutide at 2.4 mg, the dose it is actually approved for in obesity. This is tirzepatide’s top obesity dose against semaglutide’s top obesity dose. There is no dose alibi.
The result: a decisive win for tirzepatide
At 72 weeks, tirzepatide produced −20.2% mean body-weight loss versus −13.7% for semaglutide (p<0.001) — roughly 22.8 kg versus 15.0 kg. That is about a 6.5-percentage-point gap, or close to 50% more relative weight lost on tirzepatide.
The responder numbers are where the gap becomes vivid, because weight loss above 25–30% is the territory once reserved for bariatric surgery:
| Outcome at 72 weeks | Tirzepatide (10/15 mg) | Semaglutide (1.7/2.4 mg) |
|---|---|---|
| Mean body-weight change | −20.2% | −13.7% |
| Mean absolute loss | ~22.8 kg | ~15.0 kg |
| Lost ≥25% of body weight | 31.6% | 16.1% |
| Lost ≥30% of body weight | 19.7% | 6.9% |
| Mean waist-circumference change | −18.4 cm | −13.0 cm |
Roughly twice as many people hit the ≥25% threshold on tirzepatide, and nearly three times as many hit ≥30%. Waist circumference — a better proxy for the visceral fat that drives metabolic risk than weight alone — fell further too.
On tolerability the trial was reassuring for both: most adverse events were the expected mild-to-moderate gastrointestinal effects (nausea, diarrhea, constipation) clustered during dose escalation, and the overall proportion of participants reporting adverse events was similar between the two groups. Tirzepatide won on weight without obviously costing more in side effects over 72 weeks.
Why tirzepatide pulls ahead: the GIP arm
The reason is mechanistic, not incidental. Semaglutide is a single-receptor agonist — it activates the GLP-1 receptor only. Tirzepatide is a dual agonist: it hits GLP-1 and GIP (glucose-dependent insulinotropic polypeptide). Both drugs lean on the same core GLP-1 effects — slowed gastric emptying, enhanced satiety, and central appetite suppression in the hypothalamus — which is why both work at all. Adding the GIP arm appears to deepen the appetite and metabolic response rather than just duplicating it, lifting the achievable ceiling.
This is the same logic that governs the whole class: stacking complementary receptors tends to move the weight-loss ceiling upward. It is exactly why the triple agonist one tier up — retatrutide, which adds a glucagon arm to GLP-1 and GIP — posts even larger numbers than tirzepatide in its own trials (the comparison we cover in retatrutide vs tirzepatide). For the full picture of how these gut-brain-pancreas circuits translate into lost weight, see how GLP-1 works for weight loss.
What SURMOUNT-5 does not settle
A clean win on one endpoint is not a verdict on everything. Four questions survive the trial intact, and they are the ones that often decide the real choice.
Hard outcomes, not just the scale. SURMOUNT-5 measured weight, waist and metabolic markers — not heart attacks, strokes, kidney decline or deaths. This is where semaglutide still leads the entire class: dedicated cardiovascular-, kidney- and heart-failure-outcome trials have shown injectable semaglutide reduces those events, evidence tirzepatide’s outcome programs are still generating. Loses more weight and has more proof the weight loss prevents events are different claims, and they currently point at different drugs. We unpack the outcome and safety signals — and their limits — in what the 2026 data shows on GLP-1 and cancer risk.
Open-label, industry-funded. SURMOUNT-5 was open-label — participants and investigators knew which drug was being given — and it was funded by tirzepatide’s manufacturer. Neither fact invalidates a pre-registered primary endpoint of measured body weight, which is hard to bias by expectation, but both are honest caveats worth carrying.
Lean mass. Neither the headline nor this trial resolves what is lost. Aggressive appetite suppression risks muscle alongside fat, and a bigger total loss can mean a bigger absolute lean-mass loss if protein intake and resistance training don’t keep up. That risk scales with efficacy, so it arguably matters more on the winning drug — see how to keep muscle on a GLP-1.
Cost, access and the third option. The trial says nothing about price, insurance coverage, supply, or the fact that a more powerful agent (retatrutide) sits above both — investigational, but reshaping the ceiling. A clinical “best” and a practical “best for you” are not the same line.
A post-hoc analysis did push the story one step further: modeling 10-year cardiovascular risk from the measured changes, the tirzepatide group’s greater weight and waist reduction translated into a larger projected risk drop. That is a projection built on risk equations, not measured events — supportive, but not a substitute for an outcomes trial.
Where each one fits
The honest read of SURMOUNT-5 is narrow and strong at the same time:
- If the goal is the most weight loss from an approved, real-world-proven drug: tirzepatide. It won the only head-to-head, at fair max-vs-max doses, without a tolerability penalty.
- If the goal is the deepest evidence that the weight loss prevents downstream events: injectable semaglutide still owns the outcome data, and that can outweigh a percentage-point gap for higher-risk individuals.
- If raw magnitude is everything and investigational compounds are on the table: the ceiling is higher still one tier up, in the triple-agonist retatrutide Phase 3 results — but it is not approved and has no head-to-head of its own yet.
SURMOUNT-5 settled which loses more weight. It did not settle which is right for whom — and conflating those two is the most common mistake in the post-trial coverage.
FAQ
Is tirzepatide better than semaglutide for weight loss? In the only large head-to-head obesity trial, yes. SURMOUNT-5 showed tirzepatide produced 20.2% mean weight loss versus 13.7% for semaglutide at 72 weeks, with about twice as many people losing ≥25% of their body weight. That direct comparison is far stronger evidence than lining up their separate trials.
By how much did tirzepatide beat semaglutide? About 6.5 percentage points (20.2% vs 13.7%), or roughly 22.8 kg vs 15.0 kg — close to 50% more relative weight loss. On the ≥30% threshold the gap was widest: 19.7% vs 6.9%.
Was the comparison fair, or was semaglutide underdosed? It was fair. SURMOUNT-5 used semaglutide 2.4 mg, its maximum approved obesity dose, against tirzepatide’s maximum 15 mg. That is different from the earlier SURPASS-2 diabetes trial, which used a lower semaglutide dose.
Why does tirzepatide work better? Tirzepatide activates two incretin receptors (GLP-1 and GIP); semaglutide activates only GLP-1. Adding the GIP arm appears to deepen appetite suppression and metabolic effect, raising the weight-loss ceiling rather than just duplicating the GLP-1 effect.
Does this mean semaglutide is the worse drug overall? No. Semaglutide loses on weight magnitude but still leads the class in proven hard outcomes — cardiovascular, kidney and heart-failure event reductions from dedicated trials. “More weight loss” and “more outcome proof” currently describe different drugs.
Are these the same as the research peptides sold online? Tirzepatide, semaglutide and retatrutide are sold for laboratory research use only and are not approved for human use in that form. The branded medicines (Zepbound, Wegovy) are prescription pharmaceuticals. This article describes the published clinical evidence, not a protocol for personal use.
References
- Aronne LJ, Horn DB, le Roux CW, et al. Tirzepatide as Compared with Semaglutide for the Treatment of Obesity (SURMOUNT-5). N Engl J Med. 2025. doi:10.1056/NEJMoa2410819 · NCT05822830.
- Jastreboff AM, Aronne LJ, Ahmad NN, et al. Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1). N Engl J Med. 2022;387(3):205-216. doi:10.1056/NEJMoa2206038.
- Wilding JPH, Batterham RL, Calanna S, et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1). N Engl J Med. 2021;384(11):989-1002. doi:10.1056/NEJMoa2032183.
- Frías JP, Davies MJ, Rosenstock J, et al. Tirzepatide versus Semaglutide Once Weekly in Patients with Type 2 Diabetes (SURPASS-2). N Engl J Med. 2021;385(6):503-515. doi:10.1056/NEJMoa2107519.
- Lingvay I, Mosenzon O, Aronne LJ, et al. Tirzepatide compared with semaglutide and 10-year cardiovascular disease risk reduction in obesity: post-hoc analysis of SURMOUNT-5. Eur Heart J Open. 2025;5(5):oeaf117. doi:10.1093/ehjopen/oeaf117.
*Research and educational use only. This article summarizes published clinical research and is not medical advice, a treatment recommendation, or a substitute for a licensed clinician. SURMOUNT-5 was an open-label, manufacturer-funded trial; weight-loss figures are from that and other industry-sponsored studies. Tirzepatide, semaglutide and retatrutide referenced here are sold for laboratory research purposes only and are not intended to diagnose, treat, cure, or prevent any disea