Mounjaro or Wegovy — which is actually better for weight loss?

What the head-to-head trial tells us, and what it doesn't

The most direct comparison comes from SURMOUNT-5, a 72-week open-label trial published in the New England Journal of Medicine in 2025. It set tirzepatide against semaglutide 2.4mg in adults with obesity but no type 2 diabetes, and tirzepatide produced greater average weight reduction across the trial period. That result matters because it is not an indirect model or a pooled estimate, it is the two medicines running alongside each other in the same study.

Earlier individual trials give useful context. SURMOUNT-1 recorded around 20–21% average body-weight reduction at tirzepatide 15mg over 72 weeks. The STEP 1 trial, the landmark study for semaglutide 2.4mg, found roughly 15% average reduction over 68 weeks, as detailed in the published STEP 1 paper in the New England Journal of Medicine. These figures come from different populations at different points in time, so they are supporting evidence rather than a verdict on their own.

One thing worth flagging: Novo Nordisk's 7.2mg semaglutide dose, approved by the MHRA in January 2026 and available as a dedicated single-dose pen from April 2026, produced around 20.7% average weight loss in trials. That narrows the gap considerably compared to the original 2.4mg maintenance dose. So the comparison is not quite as simple as a single number suggests, it depends which dose of each medicine you are actually able to reach and tolerate.

How the two medicines work differently, and why that matters for some people

Mounjaro is a dual GIP and GLP-1 receptor agonist. It activates two separate gut-hormone pathways that influence appetite, satiety and blood-sugar regulation. Wegovy works on the GLP-1 pathway only. In practice, both slow gastric emptying and reduce appetite (the experience of feeling full sooner is common to both) but the dual mechanism is thought to account for a greater effect on body weight at equivalent tolerability in the evidence so far.

Whether that translates into a noticeably different experience for you personally is less predictable. Some people tolerate one medicine better than the other. Side-effect profiles overlap significantly: nausea, constipation, indigestion and similar gastrointestinal effects are common to both, typically at their most noticeable after a dose increase and settling over a week or two for most people. Neither medicine is free of that adjustment period.

There is also a practical difference worth knowing. For women taking oral contraceptives, NHS England's guidance on weight-management injections recommends adding a non-oral method of contraception for the first four weeks of Mounjaro treatment and after each dose increase, because absorption of the pill may be reduced. No equivalent recommendation applies to semaglutide currently. If that affects your situation, it is worth raising with your prescriber before you start.

Eligibility and access, where the two medicines sit differently

Both medicines share broadly similar private licensing criteria: adults with a BMI of 30 or above, or 27 or above alongside at least one weight-related condition such as high blood pressure, high cholesterol, type 2 diabetes or obstructive sleep apnoea. Lower BMI thresholds can apply for some ethnic backgrounds under UK guidance. Neither licence guarantees prescription, a prescriber assesses the full picture.

On the NHS, access to each is governed by separate NICE guidance with different conditions attached. Mounjaro is recommended under NICE TA1026 for adults with a BMI of 35 or above and at least one weight-related comorbidity, with access being phased in gradually by cohort. Wegovy is available under NICE TA875, but only within specialist weight management services and for a maximum of two years. Waiting lists for both NHS routes are commonly long, and not every GP practice is yet set up to prescribe under the new arrangements. You can read more about how Wegovy fits into this landscape or explore what the private route looks like on our weight-loss treatments overview.

One honest note on cost: if you are weighing up the two medicines partly on price, the context matters. Eli Lilly raised Mounjaro's UK list price from September 2025, and typical private prices for both medicines have moved since then. Our Wegovy cost guide walks through what affects pricing and what a legitimate private price should include, because the cheapest listing is rarely the right test for a prescription medicine. Worth a read before you decide.

So which should you choose?

The evidence, on balance, currently favours Mounjaro for greater average weight loss, particularly for people who can reach and tolerate higher doses. That is also the view reflected in NICE's appraisal process, where indirect comparisons have noted the tirzepatide advantage. If you want to understand the key distinctions before your consultation, our guide to what sets Mounjaro and Wegovy apart across their mechanisms, dosing and trial results is a useful place to start.

But trials measure averages across thousands of participants. Your prescriber looks at one person. Factors like your medical history, other medicines you take, whether injections feel manageable for you, and which conditions sit alongside your weight all feed into the clinical judgement. Some people do better on semaglutide; some thrive on tirzepatide; a small number do not tolerate either well enough to continue. If you are trying to work out how Mounjaro and Wegovy compare across dosing, mechanisms and real-world outcomes, our dedicated comparison covers the detail worth knowing before a consultation. There is also the question of the Wegovy tablet (the first oral GLP-1 medicine licensed for weight management in the UK, approved by the MHRA in June 2026) which suits people who strongly prefer not to inject. That is a separate conversation worth having if needles are a barrier.

If you are already on one treatment and wondering whether to switch, our team hears that question fairly often. It is not a decision to make unilaterally, and if you want a fuller picture of how the two medicines stack up, our page on whether Wegovy is better than Mounjaro works through the evidence on both sides. For anyone who has weighed up the trial data and still wants a clearer steer, our guide on whether Mounjaro is better than Wegovy looks at the question from that angle specifically. A prescriber can go through your specific situation with you properly. If you would like a clinical view on which direction suits your situation, check your eligibility with our prescribers and they will take it from there.