Mounjaro vs Wegovy: how the weight-loss injections compare

Mounjaro and Wegovy are both prescription injections used for weight management in the UK, alongside dietary and activity support. They work on related biological pathways, but they are not the same drug. The most useful comparison is not "which one wins on social media?" It is what the trials show, what the medicines ask of a patient, and which treatment a qualified prescriber considers suitable.

How they compare

Both reduce appetite and help people feel fuller. Both can cause gastrointestinal effects such as nausea, vomiting, diarrhoea and constipation. Their labels, dose schedules, contraindications and access routes are not interchangeable; a prescriber or pharmacist should check the details for the specific product.

What did the direct comparison find?

SURMOUNT-5 directly compared tirzepatide with semaglutide over 72 weeks in 751 adults with obesity or overweight plus a weight-related condition. Participants did not have diabetes and received counselling on a reduced-calorie diet and increased physical activity. The treatment-regimen estimate of average weight change was −20.2% with tirzepatide and −13.7% with semaglutide. The difference was statistically significant. The published SURMOUNT-5 report describes the population and results.

That is valuable evidence because it is a head-to-head trial, rather than a comparison of two separate studies. It still does not predict an individual result. The study used specific doses, followed participants for 72 weeks, excluded people with diabetes and included lifestyle counselling. People in routine care vary in health, starting weight, dose tolerance, treatment duration and support.

Does a bigger average loss make Mounjaro the right choice?

Not automatically. A trial average cannot capture everything that matters to a patient. One medicine may be unsuitable because of medical history, another may be better tolerated, and access, cost, continuity of supply or personal preference may affect the practical choice. A clinician should also review other medicines, pregnancy plans and conditions such as diabetes, gallbladder disease or severe gastrointestinal problems.

Do not take both medicines together or switch between them without medical advice. They affect overlapping pathways, and a switch needs a planned dose and timing. If the current medicine is not working well, discuss what "working" means: appetite, weight trend, blood glucose, side effects, mobility or another health goal.

The useful question for a consultation

Instead of asking only which drug causes more average weight loss, ask: "Which option best fits my health history and treatment goal, what support comes with it, and how will we review whether the benefits still outweigh the downsides?" That question is less dramatic, and more likely to lead to a useful decision.