The differences between Mounjaro and Wegovy: what the evidence actually shows

Two different receptors, one shared goal — what the trial data showed

Start with the published evidence. SURMOUNT-1, the pivotal tirzepatide trial reported in the New England Journal of Medicine, followed 2,539 adults with obesity over 72 weeks. At the 15 mg maintenance dose, participants lost around 20–21% of body weight on average. The STEP 1 trial, also published in the NEJM, tested semaglutide 2.4 mg over 68 weeks and found an average reduction of roughly 15%. Those figures matter, but they come with an important caveat: the two trials recruited slightly different populations at different time points, so they are not a direct head-to-head.

A proper head-to-head arrived with SURMOUNT-5, an open-label trial published in 2025 in the New England Journal of Medicine. Across 751 adults with obesity and no diabetes, followed for 72 weeks, tirzepatide produced a greater average weight reduction than semaglutide 2.4 mg. Novo Nordisk subsequently gained MHRA approval for a higher 7.2 mg semaglutide dose (one weekly injection) with 72-week trial data showing around 20.7% average loss, narrowing the gap considerably. You can read about the trial landscape and what it means for treatment decisions on our Wegovy vs Mounjaro overview.

The underlying mechanism explains some of the difference. Mounjaro (tirzepatide, made by Eli Lilly) is the only dual GIP and GLP-1 receptor agonist licensed for weight management in the UK, and if you are weighing up your options, our page covering what's the difference between Wegovy and Mounjaro walks through how those mechanisms translate into real-world treatment choices. Wegovy (semaglutide, made by Novo Nordisk) acts on GLP-1 receptors alone. Both slow gastric emptying and reduce appetite; tirzepatide's dual action appears to amplify those effects in most people. Neither is a substitute for the other, they are distinct medicines with distinct profiles.

Who each medicine is licensed for, and where UK eligibility guidance differs

NICE TA1026 recommends tirzepatide for adults with a BMI of 35 or above plus at least one weight-related comorbidity. Semaglutide is covered by NICE TA875 under broadly similar BMI criteria, but only for a maximum of two years and only within a specialist weight management service with multidisciplinary support. Both appraisals apply lower BMI thresholds for some ethnic backgrounds, 2.5 kg/m² lower for South Asian, Chinese, Middle Eastern, Black African or African-Caribbean backgrounds.

For private prescribing the licensed criteria are broader: both medicines are licensed for adults with a BMI of 30 or above, or 27 or above with at least one weight-related condition such as hypertension, type 2 diabetes, dyslipidaemia or obstructive sleep apnoea. A prescriber assesses the whole clinical picture, BMI alone is not a rubber stamp. If you want to see where you might sit before booking a consultation, our weight-loss treatment overview sets out the licensed eligibility criteria in plain terms.

One thing worth being clear about: Ozempic is also semaglutide, but it is licensed for type 2 diabetes, not weight management. Prescribing it for weight loss would be off-label. The differences between Ozempic and Mounjaro are covered separately; this page focuses on the two medicines that are actually licensed for weight loss in the UK. Similarly, Zepbound is the US brand name for tirzepatide and is not available as a UK product, if you have seen it mentioned, our page on Wegovy versus Zepbound explains the distinction.

Side-effect profiles and practical differences you'll notice day to day

Both medicines share a broadly similar side-effect pattern because both act on the gut: nausea, vomiting, diarrhoea, constipation, indigestion and fatigue are the most commonly reported effects with each. These tend to be most noticeable in the first weeks of treatment or after a dose increase, and they often settle as your body adjusts. If you are still deciding between the two and want a closer look at how the side-effect profiles compare in practice, our guide on what's the difference between Mounjaro and Wegovy covers this alongside the clinical detail from the NHS tirzepatide medicines page and the equivalent semaglutide page.

In January 2026 the MHRA issued a Drug Safety Update reminding prescribers and patients that acute pancreatitis, though infrequent, is a known risk with GLP-1 medicines. Severe, persistent stomach pain that reaches through to the back (with or without vomiting) warrants urgent medical attention. That applies to both Mounjaro and Wegovy.

Practically speaking, both medicines arrive as once-weekly subcutaneous injections. Mounjaro comes as a pre-filled KwikPen, each pen contains four weekly doses. Wegovy uses individual pre-filled pens. Both need refrigeration between 2–8°C; the Patient Information Leaflet for each product sets out the exact room-temperature window you have once a pen is out of the fridge, and that is where to look rather than any online summary. One thing patients often mention: when the DPD notification arrives and they open a plain, unmarked box for the first time, the pen is smaller and more discreet than they expected. For cost context across providers, our Wegovy vs Mounjaro cost comparison covers what private pricing typically includes and excludes.

Which one is right for you?

The trial numbers give a useful steer, but they describe population averages across thousands of participants, they do not predict what will happen for any one person. Factors that a prescriber weighs include your current BMI and trajectory, any existing conditions, your medication list (oral contraceptive users starting tirzepatide, for example, are advised to add a non-oral method for the first four weeks of treatment and after each dose increase, because pill absorption may be affected), and how you have responded to any previous treatment.

Our clinical team, led by our clinical pharmacist prescribers, reviews every consultation personally. There is no algorithm making the call. If you have questions before you start, our FAQs cover the most common ones, and you can reach our team directly via our contact page. When you are ready, the starting point is the same for both medicines: start your free consultation and speak to our prescribers about which option makes clinical sense for you.