Mounjaro®
Starting from £179.99/mo
Start journey Learn moreHow the Trial Percentages Compare, and What They Actually Mean for You
You've seen the numbers — here's the full picture behind them
Imagine you're sitting with a printout of two trial results, trying to work out which medicine gives the better outcome. The SURMOUNT-1 trial of tirzepatide, published in the New England Journal of Medicine, reported average weight reductions of around 20–21% at the 15mg dose over 72 weeks. The STEP 1 trial of semaglutide 2.4mg, also published in the New England Journal of Medicine, reported around 15% over 68 weeks. Both figures come from large, rigorous, randomised controlled trials in adults with obesity and without type 2 diabetes.
Those weren't different methodologies producing incomparable numbers. But they also weren't a head-to-head test. The populations differed slightly, the trial durations differed slightly, and the comparison is indirect. That indirect gap was narrowed (though not eliminated) when researchers ran SURMOUNT-5, a direct open-label head-to-head trial of tirzepatide versus semaglutide 2.4mg, published in the New England Journal of Medicine in 2025. Tirzepatide again produced greater average weight loss. NICE referenced these findings in its committee discussions for tirzepatide's appraisal, TA1026, published in December 2024.
The practical upshot: on current evidence, tirzepatide's average percentage weight loss is higher. By how much varies by dose and individual. Neither figure is a promise; both are population averages from tightly controlled conditions alongside diet and activity support.
What the percentage gap looks like in a table, and where the data comes from
It helps to lay the key figures side by side. A fuller breakdown of how these two medicines compare across several dimensions is available on our Mounjaro vs Wegovy for weight loss page, but the core weight-loss data is here.
| Factor | Tirzepatide (Mounjaro) | Semaglutide 2.4mg (Wegovy) |
|---|---|---|
| Average weight loss at highest dose | ~20–21% (72 weeks, SURMOUNT-1) | ~15% (68 weeks, STEP 1) |
| Head-to-head result | Greater average loss (SURMOUNT-5, 2025) | Lower average loss in the same trial |
| UK brand name | Mounjaro (Eli Lilly) | Wegovy (Novo Nordisk) |
| Mechanism | Dual GIP and GLP-1 receptor agonist | GLP-1 receptor agonist |
| Higher-dose option | Up to 15mg weekly | Up to 7.2mg weekly (approved April 2026) |
| NICE-recommended (England) | Yes, TA1026 (Dec 2024) | Yes, TA875 (Mar 2023), max 2 years in specialist services |
One important note on the 7.2mg semaglutide dose: the MHRA approved a dedicated single-dose 7.2mg Wegovy pen in April 2026, with trial data reporting around 20.7% average weight loss over 72 weeks, narrowing the gap with tirzepatide's highest dose considerably. Our page on whether Wegovy is as good as Mounjaro covers that emerging picture in more detail.
Why the percentage isn't the only question worth asking
A question our prescribers hear regularly: "If tirzepatide's percentage is higher, shouldn't everyone just take that?" It's a reasonable instinct, and not quite the right one. The percentage from a trial tells you what happened on average. Your response depends on your starting weight, your metabolic profile, any conditions you already have, which medicines you're already taking, and how your body tolerates the titration schedule.
Side-effect tolerance matters here. Both medicines share a GI-led side-effect profile: nausea, loose stools, constipation and reflux are the most commonly reported, typically most noticeable at the start or after a dose increase before settling. Neither medicine suits everyone equally well. Some people do better on one than the other for reasons that aren't always predictable in advance. If you're thinking about switching, our guide to moving from semaglutide to tirzepatide covers what that involves clinically.
There's also the question of what you're comparing against in the first place. Ozempic is also semaglutide, but it's licensed for type 2 diabetes, not weight management, and if you're wondering whether tirzepatide is the same as semaglutide, it isn't, and that page explains the differences in mechanism and approval. The licensed weight-loss medicines in the UK are Mounjaro and Wegovy. Worth keeping that straight before drawing conclusions from numbers you've seen online.
What this means if you're deciding between the two
The NHS has its own criteria for each medicine (strict phased eligibility thresholds under NICE TA1026 for tirzepatide and specialist-service requirements for Wegovy under TA875) and access on the NHS can involve a considerable wait. Privately, eligibility is broader: adults with a BMI of 30 or above, or 27 with a weight-related condition such as high blood pressure or raised cholesterol, can be assessed without a referral. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance.
The percentage figures in the trials are a useful starting point for a conversation, not a finishing line. Which suits you is a clinical decision our prescribers make with you, taking your full health picture into account, not a league table read-off. If you'd like that conversation, you can check your eligibility and start a free consultation with the nume team today. Our clinical team's background is on the clinical team profile page if you'd like to know more about who reviews your case.
For a broader look at how the two medicines sit alongside each other, our semaglutide or tirzepatide overview is a good next read. And if cost is part of your thinking, the Mounjaro and Wegovy price comparison lays out what you'd typically be looking at in the current UK private market.
Looking to start your weight loss journey?
Take a quick eligibility quiz to explore your options and see how we can support you.