Mounjaro Compared to Other Weight-Loss Medicines: What the Evidence Shows

What the head-to-head trial actually found when comparing Mounjaro and semaglutide

For years, direct comparison between tirzepatide and semaglutide relied on indirect cross-trial analysis, useful but imperfect. SURMOUNT-5, published in the New England Journal of Medicine in 2025, changed that by randomising 751 adults with obesity but without diabetes to either tirzepatide or semaglutide 2.4mg weekly for 72 weeks. Tirzepatide produced greater average body-weight reduction. That's a clean result from a rigorous design, and it's the benchmark clinicians use when they compare Mounjaro to Wegovy today.

The reason the medicines perform differently comes down to mechanism. Semaglutide targets the GLP-1 receptor, which slows gastric emptying and signals fullness through the brain. Tirzepatide does that too, but also activates the GIP receptor, a second gut-hormone pathway involved in appetite and energy regulation. More detail on how that dual action works is on the tirzepatide overview page. Whether the second pathway is what drives the difference, or whether dose and individual response matter more, is still being studied, but the weight-loss outcomes in trials favour tirzepatide across doses tested.

One important caveat: SURMOUNT-5 compared tirzepatide against semaglutide 2.4mg. The MHRA approved a 7.2mg maintenance dose of semaglutide (Wegovy) for adults with BMI ≥30 in early 2026, with trials reporting around 20.7% average weight loss at that dose. That narrows the gap considerably. Specific dose availability is confirmed at consultation, not guaranteed in advance.

How Mounjaro compares to older oral treatments like orlistat

Orlistat occupies a different part of the landscape entirely. Rather than acting on gut hormones or appetite signalling, it inhibits pancreatic lipase, blocking the absorption of roughly a third of dietary fat from each meal. The NHS orlistat page is clear that it produces meaningful weight loss for some people when combined with a reduced-calorie diet, but the average outcomes in clinical trials are substantially more modest than those seen with injectable GLP-1 or dual-agonist treatments.

The practical differences are significant too. Orlistat is taken three times daily with meals and requires a consistently low-fat diet to avoid its well-documented gastrointestinal side effects. Mounjaro is a once-weekly injection. That's not a reason to dismiss orlistat, for people who prefer an oral route, can't tolerate injectables, or have conditions that make GLP-1 medicines unsuitable, it remains a legitimate option. But when you're doing a like-for-like compare across weight-loss medicines licensed in the UK today, the efficacy data at population level does not put them in the same bracket.

If you want a detailed look at how providers and treatments differ, the Mounjaro comparison guide covers a wider range of factors, including what varies between private services.

NICE recommendations: what the approval criteria reveal about where each medicine sits

NICE appraisals are useful not just as access criteria but as a statement about the evidence the regulator found credible. NICE recommended tirzepatide for adults with a BMI ≥35 and at least one weight-related comorbidity (TA1026, December 2024), reducing the threshold by 2.5 kg/m² for South Asian, Chinese, Middle Eastern, Black African or African-Caribbean backgrounds. The recommendation for semaglutide (TA875, 2023) came with an additional condition: use for a maximum of two years, within a specialist weight management service.

That two-year cap on semaglutide reflects the NICE committee's view of the evidence at the time of appraisal; the tirzepatide recommendation does not carry the same restriction. It isn't a signal that one medicine is safer, but it is a meaningful difference in how NICE has framed the two. You can read the full NICE appraisal of tirzepatide (TA1026) if you want to trace the committee's reasoning directly.

For more context on how private prices for Mounjaro factor into this decision, see the Mounjaro pricing guide. The short version: price is rarely the right variable to optimise for when comparing prescription medicines.

What a comparison like this can't tell you

Population-level averages are a starting point, not a verdict. Individuals vary enormously: someone who loses 8% on tirzepatide and 22% on semaglutide isn't a statistical anomaly, they're a person with a different biology. Side-effect tolerance matters too, Mounjaro and Wegovy share a broadly similar GI profile (nausea, bloating, changes in bowel habit, fatigue), but individuals experience them differently, and that can influence which medicine is sustainable. There's a fuller breakdown of what to watch for on the Mounjaro and wind/GI effects page.

None of this means comparison is pointless. It means the comparison is a starting point for a conversation with a prescriber, not a substitute for one. A prescriber reviews your health history, current medications, BMI, comorbidities and personal circumstances, the kind of picture that a comparison table, however thorough, can't capture. The Mounjaro comparison website guide covers what to check when evaluating any provider you're considering, including a breakdown of the companies currently offering Mounjaro in the UK and how they differ.

If you'd like that conversation now, speak to our prescribers, consultations are free, clinically reviewed on the same day by a GPhC-registered Independent Prescriber, and there's no obligation.