Mounjaro or semaglutide for weight loss: what the evidence actually shows

Step 1 — understand what each medicine actually does

Semaglutide (the active ingredient in Wegovy) is a GLP-1 receptor agonist. It mimics a gut hormone released after eating, slowing gastric emptying, blunting appetite and nudging blood-sugar regulation. It acts on one receptor pathway. Tirzepatide (Mounjaro) takes a different approach: it activates both the GLP-1 and GIP receptors, making it the only dual-agonist weight-loss medicine licensed in the UK. GIP is a second gut hormone with overlapping but distinct effects on appetite and fat metabolism. Whether that second pathway translates into meaningfully greater weight loss for any individual person is exactly what the clinical trials set out to measure, and the answer is broadly yes, on average, though not for everyone and not by the same margin across doses. An overview of the UK-licensed weight-loss treatments puts both medicines in broader context if you want to start there. The NHS publishes straightforward patient-level information on both tirzepatide and semaglutide that is worth reading before any consultation.

Step 2, look at what the trial results show

In SURMOUNT-1, a 72-week randomised controlled trial involving 2,539 adults with obesity and no type 2 diabetes, tirzepatide at its highest dose produced average body-weight reductions of around 20–21%, with some analyses reaching approximately 22.5%. The STEP 1 trial of semaglutide 2.4mg (the standard Wegovy maintenance dose for most of the time it has been available) showed an average reduction of around 15% over 68 weeks. Those figures come from different trials with different populations, so they are not a clean head-to-head. The cleaner comparison came from SURMOUNT-5, a 72-week open-label trial published in the New England Journal of Medicine in 2025, which randomised 751 adults with obesity directly to tirzepatide or semaglutide 2.4mg. Tirzepatide produced greater average weight loss. It is worth knowing that semaglutide's gap to tirzepatide narrows at the newer 7.2mg dose, approved by the MHRA in early 2026, and if you want a thorough look at whether semaglutide or tirzepatide comes out ahead across the full trial evidence, that page works through the data in detail. A detailed Wegovy versus Mounjaro breakdown covers the SURMOUNT-5 numbers more fully. NICE's appraisal of tirzepatide (TA1026) and its equivalent for semaglutide (TA875) are the authoritative UK assessments of the evidence.

Step 3 (compare the practical details side by side

One practical note on Wegovy: Novo Nordisk also makes Ozempic, which contains the same active ingredient. Ozempic is licensed for type 2 diabetes, not weight loss, and should never be used interchangeably with Wegovy for weight management. If you have seen references to Ozempic in a weight-loss context, this page explains the distinction clearly.

Step 4, think about eligibility and the prescriber's role

On paper, the licensed eligibility criteria for both medicines are similar: 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, obstructive sleep apnoea, prediabetes or type 2 diabetes. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance. That shared starting point is where the similarity ends for clinical decision-making. A prescriber will consider your existing conditions, any medicines you already take, your history of GI symptoms, whether you have tried either class before, and your own preferences, including, frankly, whether you have any anxiety about injections. Some people manage very well on semaglutide for years; others reach a plateau and a conversation about switching becomes relevant, and patients often find it helpful to read about whether Mounjaro or semaglutide is the better fit before that discussion with their prescriber. Switching between the two is something our prescribers discuss with patients regularly. If you are already on a treatment elsewhere and want a second clinical view, nume, sorry, our clinical team reviews transfer patients before dispensing anything. The right choice here is genuinely a clinical one. Start your free consultation and a prescriber will work through it with you, and if you are still weighing up the options beforehand, our page on what is better, semaglutide or tirzepatide, covers the key considerations in plain language, many people find that conversation happens faster than they expected, even if they place an order on a Monday or just after a bank holiday.