Mounjaro®
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Start journey Learn moreHow the evidence stacks up: trial results, mechanism differences and what they mean for you
What the SURMOUNT-5 trial found when the two medicines went head to head
The clearest data we have comes from the SURMOUNT programme published in the New England Journal of Medicine, and from SURMOUNT-5 specifically, a 72-week open-label trial in 751 adults with obesity but without type 2 diabetes. Tirzepatide at its highest tolerated dose produced meaningfully greater average weight reduction than semaglutide 2.4mg over the same period. NICE's appraisal notes in technology appraisal TA1026 that indirect comparisons also favour tirzepatide, though they acknowledge the limitations of comparing across separate trial populations. The STEP 1 trial for semaglutide 2.4mg (68 weeks, adults without diabetes) showed roughly 15% average body-weight reduction. SURMOUNT-1, testing tirzepatide, showed around 20–21% at the 15mg dose. A gap exists. It is real. Whether it matters for any individual person is a different question.
The newer semaglutide 7.2mg dose, approved by the MHRA in early 2026, narrows that gap considerably, trials reported around 20.7% average loss over 72 weeks at that higher maintenance level, approaching tirzepatide 15mg territory. So the landscape is still shifting. You can read more about how the head-to-head data was constructed on our SURMOUNT-5 trial breakdown page.
One thing both sets of trials share: results vary enormously between individuals. Trial averages describe populations, not predictions for a specific person.
The mechanism gap, one receptor versus two
Semaglutide (Wegovy) activates the GLP-1 receptor: it slows gastric emptying, signals fullness to the brain, and reduces appetite. Tirzepatide (Mounjaro) does all of that and also activates the GIP receptor, making it the only dual-agonist weight-loss medicine currently licensed in the UK. Whether the second receptor pathway is the reason for the larger average weight loss seen in trials is still a live question in the research literature, but that is the working hypothesis. The NHS tirzepatide medicine page explains the dual mechanism in plain terms if you want the patient-level overview.
Practically, both medicines slow the rate at which food leaves your stomach. That is why the side-effect profiles overlap: nausea, some loose stools, constipation, indigestion and fatigue are common to both, typically most noticeable after starting or moving up a dose, and usually settle within days to a couple of weeks. The frequency and severity can differ between people, sometimes significantly. A conversation about tolerability is a normal part of the prescriber review, our page on Wegovy versus Mounjaro goes into the side-effect comparison in more detail.
Comparing the two medicines at a glance
| Factor | Tirzepatide (Mounjaro) | Semaglutide (Wegovy injection) |
|---|---|---|
| Mechanism | Dual GIP + GLP-1 receptor agonist | GLP-1 receptor agonist |
| Injection frequency | Once weekly | Once weekly |
| UK licensed for weight management | Yes (BMI ≥30, or ≥27 with a weight-related condition) | Yes (BMI ≥30, or ≥27 with a weight-related condition) |
| Average weight loss in trials | ~20–21% at 15mg (SURMOUNT-1, 72 weeks) [NEJM] | ~15% at 2.4mg (STEP 1, 68 weeks); ~20.7% at 7.2mg [NEJM; MHRA] |
| NICE appraisal | TA1026 (published Dec 2024) | TA875 (published Mar 2023) |
| Oral option available? | No (injection only) | Yes, Wegovy tablet approved by MHRA June 2026 |
Two medicines that are not licensed for weight loss often come up in the same searches: Ozempic is also semaglutide, but its UK licence covers type 2 diabetes, not weight management. Rybelsus is oral semaglutide, again for diabetes. Prescribing either for weight loss would be off-label. If you've seen them mentioned alongside the licensed options, our Mounjaro vs Ozempic page clarifies exactly where each one sits.
Cost context, eligibility and how to make an actual decision
Private pricing for both medicines has moved over the past year. Eli Lilly raised Mounjaro's UK list price from September 2025; as widely reported, the higher doses now carry list prices substantially above where they started, with typical private prices ranging roughly £149–£375 per month by dose and provider. Semaglutide pricing has followed a similar pattern. Cost matters, but for a prescription medicine it is the wrong place to start the comparison, our cost comparison page looks at this honestly.
On the NHS, access to both medicines is subject to strict phased criteria. Tirzepatide became available in primary care from April 2026, but only for people meeting specific BMI and comorbidity thresholds under NICE TA1026. Wegovy is available via specialist weight management services under TA875, with commonly long waiting lists. Most people reading this comparison are looking at the private route, through a regulated pharmacy, with a valid prescription following clinical assessment.
If you're weighing up whether switching makes sense, the switching between these two medicines page covers what a prescriber typically considers. And if you'd like to explore both options with someone who knows the evidence properly, the best next step is a free consultation. Which medicine suits you is a clinical decision our prescribers make with you, not one a comparison page can make for you, and our full tirzepatide vs semaglutide guide is a good place to read further before that conversation. Start your free consultation and a GPhC-registered prescriber will review your case the same day.
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