Mounjaro®
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Start journey Learn moreThe kg difference between tirzepatide and semaglutide: what the trials show, and what it means for you
Step 1, Start with the best available evidence: the SURMOUNT-5 head-to-head trial
For years, clinicians and patients were comparing tirzepatide and semaglutide across separate trials run in different populations under different conditions. That changed in 2025. The SURMOUNT-5 trial enrolled 751 adults with obesity and no diabetes, randomised them to either tirzepatide or semaglutide 2.4 mg, and followed them for 72 weeks, giving us the clearest direct comparison we have. The results, published in the New England Journal of Medicine, showed tirzepatide produced meaningfully greater average weight loss than semaglutide at the doses compared. The absolute difference in kilograms varied by analysis, but across the trial tirzepatide participants lost roughly 5 to 8 kg more on average, a gap that is clinically relevant, not just statistical noise. Our detailed breakdown of the SURMOUNT-5 findings walks through the methodology if you want the full picture. The key takeaway here: this is the first robust head-to-head, and it favours tirzepatide on average weight lost in kilograms. That said, "on average" is doing real work in that sentence. Some participants on semaglutide lost more than some on tirzepatide. Individual response varies, and no trial average predicts your result.
Step 2, Understand the earlier trial numbers and what they add
Before SURMOUNT-5, the comparison relied on separate phase 3 programmes. SURMOUNT-1 (tirzepatide, 72 weeks) reported average body-weight reductions of around 20–21% at the 15 mg dose, equivalent to roughly 22–23 kg in participants whose baseline weight was around 105–110 kg. The STEP 1 trial (semaglutide 2.4 mg, 68 weeks) reported approximately 15% average reduction, around 15–16 kg in a similar weight bracket. On those separate-trial numbers, the gap in kilograms looks even wider than SURMOUNT-5 suggests; the difference partly reflects different populations and trial designs rather than purely the medicines. A question our prescribers hear most weeks is whether the new higher semaglutide dose narrows that gap. The short answer is: yes, somewhat. The MHRA approved a 7.2 mg semaglutide weekly dose (Wegovy) in January 2026, with trials reporting around 20.7% average weight loss over 72 weeks, approaching tirzepatide 15 mg territory. Our Wegovy vs Mounjaro comparison covers this newer data in more depth. For now, the practical point is that the kg gap between the two medicines at standard licensed doses remains real, but is narrowing as semaglutide's ceiling rises.
Step 3, Read the comparison table, then the caveats
The table below summarises the key numbers from the best available evidence. Where figures come from separate trials rather than SURMOUNT-5, that is noted, and NICE's own appraisal of tirzepatide (TA1026) acknowledges that indirect comparisons favour tirzepatide on weight loss outcomes.
| Factor | Tirzepatide (Mounjaro) | Semaglutide 2.4 mg (Wegovy) |
|---|---|---|
| Mechanism | Dual GIP + GLP-1 receptor agonist | GLP-1 receptor agonist |
| Average weight loss at highest dose (separate phase 3 trials) | ~20–21% body weight (SURMOUNT-1, 72 wks) | ~15% body weight (STEP 1, 68 wks) |
| Head-to-head kg difference (SURMOUNT-5, 72 wks) | Approximately 5–8 kg more lost on average | Reference arm |
| Higher dose available | Up to 15 mg weekly | Up to 7.2 mg weekly (MHRA approved Jan 2026) |
| UK brand name | Mounjaro | Wegovy |
| UK licence: weight management | Yes (BMI ≥30, or ≥27 with a weight-related condition) | Yes (BMI ≥30, or ≥27 with a weight-related condition) |
Two things the table cannot show: tolerability and individual biology. Both medicines share a similar gastrointestinal side-effect profile (nausea, loose stools, reduced appetite) that is most noticeable early in treatment or after a dose step up. For some people that profile is easier on one medicine than the other. There is no reliable way to predict which, in advance, from a number alone. Worth keeping in mind.
Step 4, Clarify what these medicines are (and what they are not)
Both Mounjaro and Wegovy are prescription-only medicines. Neither is available to buy without a prescription, and neither should be. Ozempic is also semaglutide but is licensed for type 2 diabetes management, not weight loss; it is not the same product as Wegovy and should not be treated as a cheaper alternative. Our Mounjaro vs Ozempic page explains that distinction in full. Similarly, tirzepatide is sold in the US under the brand name Zepbound, that product does not exist in the UK; in the UK it is Mounjaro, and that is the only route. If you are trying to work out which medicine makes sense for your situation (including whether the kg difference justifies tirzepatide over semaglutide given your own health picture) that is precisely the kind of question the prescriber assessment at consultation is designed to answer. The numbers here are population averages. Your prescriber's job is to apply them to you, not to you-in-a-trial. See our full comparison of tirzepatide vs semaglutide for weight loss for more on how the clinical decision is made, and the cost difference between the two medicines if that is also a factor you are weighing. Which suits you is a clinical decision our prescribers make with you.
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