Mounjaro Versus Wegovy for Weight Loss: How Do They Compare?

Do they work the same way, or is there a real biological difference?

Wegovy contains semaglutide, a GLP-1 receptor agonist made by Novo Nordisk. It mimics one gut hormone involved in appetite regulation and slows gastric emptying. Mounjaro contains tirzepatide, made by Eli Lilly, and activates two receptors (GLP-1 and GIP) making it the only dual-agonist weight-loss medicine currently licensed in the UK. That extra pathway appears to matter in practice, not just on paper. Whether the dual mechanism genuinely explains the larger trial results is still an active area of research, but the clinical outcomes are consistent across the evidence reviewed by NICE in its appraisal of tirzepatide (TA1026).

Wegovy is injected once weekly, starting at 0.25mg and titrating to a 2.4mg maintenance dose over roughly five months. An approved 7.2mg dose now exists for eligible adults with obesity (BMI ≥30). Mounjaro also involves once-weekly injections, beginning at 2.5mg (a dose designed to let your system adjust) and stepping up in stages toward a maximum of 15mg. The pace of each titration is set by your prescriber, not by a fixed calendar.

For a more detailed look at the mechanism side of this question, the tirzepatide versus semaglutide comparison explores the pharmacology in more depth.

What does the trial evidence actually show?

Both medicines have been tested in large, placebo-controlled trials. In STEP 1 (NEJM, 2021), semaglutide 2.4mg produced around 15% average body-weight reduction over 68 weeks in adults with obesity but without diabetes. In SURMOUNT-1 (NEJM, 2022), tirzepatide at 15mg produced around 20–21% average reduction over 72 weeks in a similar population, with some analyses reaching 22.5%. Those are not directly comparable trials, so the SURMOUNT-5 study matters: it put tirzepatide head-to-head against semaglutide 2.4mg in adults with obesity over 72 weeks, and tirzepatide produced a greater average weight reduction. That finding informed NICE's discussion notes accompanying TA1026.

The table below summarises the key trial figures side by side.

A practical check worth doing before your consultation: look up your current BMI using the NHS BMI calculator, it takes under a minute and the number directly shapes which options a prescriber can consider for you.

Who qualifies for each, and is the eligibility the same?

The licensed eligibility for private prescription is broadly similar: adults with a BMI of 30 or above, or a BMI of 27 or above alongside a weight-related condition such as high blood pressure, raised cholesterol, type 2 diabetes or obstructive sleep apnoea. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance. BMI alone is never a guarantee of approval, the prescriber looks at the full clinical picture, your history and any medicines you already take.

The NICE-recommended NHS criteria differ between the two, and both are more restrictive than the licensed indications. Under TA1026, NICE recommends tirzepatide for adults with a BMI of 35 or above plus at least one weight-related comorbidity. Under TA875, NICE recommends semaglutide (Wegovy) for use within a specialist weight management service, for a maximum of two years, with similar BMI thresholds. NHS access is phased and waiting lists for specialist services are commonly long. Private treatment through a regulated pharmacy is the practical alternative for people who don't meet the NHS criteria or prefer not to wait.

If the Wegovy versus Mounjaro eligibility question is specifically what you're working through, that page goes further on the NICE thresholds.

Are the side effects different enough to matter when choosing?

In both cases the most common side effects are gastrointestinal, nausea, loose stools, constipation, acid reflux, and sometimes fatigue or headache, particularly after starting or stepping up a dose. These typically settle within a week or two for most people. The profile is similar enough that it rarely drives the choice between the two medicines on its own.

A few practical differences are worth knowing. For tirzepatide specifically, women taking an oral contraceptive pill should use an additional non-oral contraceptive method during the first four weeks of treatment and for four weeks after each dose increase, because absorption of the pill may be reduced; NHS England's guidance on weight-management injections notes this applies to tirzepatide but that equivalent evidence of reduced pill effectiveness for semaglutide has not emerged. Both medicines carry a Black Triangle (▼) designation from the MHRA, meaning they are subject to additional monitoring. Any suspected side effects can be reported at yellowcard.mhra.gov.uk. Neither medicine is recommended during pregnancy, for those trying to conceive, while breastfeeding, or for anyone under 18.

For a side-by-side look at cost as well as clinical profile, the tirzepatide versus semaglutide cost page covers the pricing context in detail. And if Ozempic has come up in your research, it is worth knowing that Ozempic (also semaglutide) is licensed for type 2 diabetes, not weight loss, a distinct situation from the Wegovy versus Mounjaro question.

Which of these two medicines suits your health picture is a clinical decision our prescribers make with you. If you are still weighing up which option fits your situation, our guide to Mounjaro versus Wegovy walks through the key differences in one place, and our semaglutide versus Mounjaro comparison is useful if you want to look at this from the semaglutide side first. Speak to our prescribers through a free consultation and we'll review your case the same day.