The difference between Wegovy and Mounjaro: what clinical trials and UK licensing tell us

Two medicines, two different receptor strategies

The most fundamental difference between Wegovy and Mounjaro sits at the molecular level. Wegovy contains semaglutide, a GLP-1 receptor agonist made by Novo Nordisk. It mimics the gut hormone GLP-1, signalling fullness to the brain, slowing gastric emptying and reducing appetite. Mounjaro contains tirzepatide, made by Eli Lilly, and it activates both the GLP-1 receptor and the GIP receptor. GIP is a second gut hormone involved in energy regulation, and adding it to the picture appears to produce a stronger appetite-suppressing effect in most people who respond well. If you want a closer look at the main difference between Mounjaro and Wegovy, that dual-agonist mechanism is where the story begins. That dual action is why tirzepatide is sometimes described as the only dual-agonist weight-loss medicine currently licensed in the UK.

Both are once-weekly subcutaneous injections using pre-filled pens, and both slow the rate at which the stomach empties after a meal. The experience of injecting is broadly similar for most patients. When your treatment arrives (the pen inside a plain DPD-tracked box, no branding visible from the outside) the administration technique is comparable: a short weekly injection into the abdomen, thigh or upper arm, rotating sites each time. The side-effect profiles overlap too, which the comparison table below reflects. You can read more about how the two medicines sit relative to each other on the Mounjaro vs Wegovy overview.

What the trial data says about average weight loss

The headline figures come from separate trials rather than a single controlled comparison. SURMOUNT-1 (tirzepatide, 72 weeks) reported average body-weight reductions of around 20–21% at the 15mg maintenance dose. The STEP 1 trial of semaglutide 2.4mg (published in the New England Journal of Medicine) reported around 15% average reduction over 68 weeks. These are different populations, different trial designs and different durations, so direct arithmetic comparison overstates certainty.

A more controlled picture came from SURMOUNT-5, an open-label head-to-head trial published in 2025. Over 72 weeks, adults with obesity and no diabetes were randomised to tirzepatide or semaglutide 2.4mg. Tirzepatide produced greater average weight reduction. The NICE committee's discussion notes for TA1026 (tirzepatide) acknowledge that indirect comparisons favour tirzepatide, while stressing that individual response varies considerably. One person's 22% on semaglutide is another's 12% on tirzepatide. The trial averages are useful context; they are not a forecast for any individual.

Wegovy's own pipeline has also moved. The MHRA approved a 7.2mg maintenance dose of semaglutide in January 2026, with trials reporting around 20.7% average loss over 72 weeks at that higher dose, narrowing the gap considerably compared with the original 2.4mg maintenance data. The clinical picture is moving, which is another reason the choice is best made with a prescriber rather than a comparison article.

UK eligibility and licensing: where the two medicines align and differ

Both Wegovy and Mounjaro are licensed for adults with a BMI of 30 or above, or 27 or above alongside at least one weight-related condition such as high blood pressure, type 2 diabetes, dyslipidaemia or obstructive sleep apnoea. Neither is licensed for use in people under 18, during pregnancy or breastfeeding, or while trying to conceive. A prescriber will review your full medical history before recommending either.

The NICE recommendations differ in their scope. NICE recommended tirzepatide in December 2024 (TA1026) for adults with a BMI of 35 or above and at least one weight-related comorbidity, with NHS rollout phased across qualifying groups through to 2027. NICE recommended semaglutide (TA875) only within specialist weight management services for a maximum of two years. Neither NICE recommendation covers private prescribing directly (they govern NHS commissioning) but they reflect the strength of evidence regulators found acceptable for each medicine. Lower BMI thresholds apply for some ethnic backgrounds under both sets of UK guidance, and your prescriber will apply the appropriate threshold to your assessment.

Ozempic is also semaglutide but holds a UK licence for type 2 diabetes management, not weight loss. Presenting it as a weight-loss option, or obtaining it for that purpose, falls outside its licensed indication. The Ozempic versus Mounjaro page covers that distinction in detail.

Side effects, contraception and a few practical differences

The side-effect profiles of both medicines are dominated by gastrointestinal symptoms: nausea, loose stools, constipation, reflux, burping and fatigue feature on both lists. These are usually most noticeable when starting treatment or after a dose increase, and they tend to ease over days to a couple of weeks. The January 2026 MHRA Drug Safety Update highlighted acute pancreatitis as an infrequent but serious risk with GLP-1 medicines generally; the advice is to seek urgent medical attention for severe, persistent stomach pain that radiates to the back.

One practical difference worth noting: women taking oral contraceptives should add a non-oral method of contraception for the first four weeks of tirzepatide treatment and for four weeks after each dose increase, because tirzepatide's effect on gastric emptying may reduce pill absorption. NHS guidance does not flag the same requirement for semaglutide. If you are on the pill, this is worth discussing with your prescriber before starting Mounjaro in particular, and our page covering what's the difference between Wegovy and Mounjaro goes into further detail on practical contrasts like this one. Questions about surgery or HRT alongside either medicine are also worth raising at consultation, our FAQ page covers common practical questions.

On cost, the two medicines sit in a similar bracket in the private market, though prices vary by dose and provider. The cost comparison page covers that in more depth. The short version is that for a prescription medicine, the right question is clinical suitability and supply-chain legitimacy, not which entry-level price is lower.

Which medicine suits you is a clinical decision our prescribers make with you, based on your BMI, medical history, any medicines you already take and what you want from treatment. Patients who want a detailed clinical breakdown before their consultation may find it helpful to read through what's the difference between Mounjaro and Wegovy before speaking with a prescriber. Check your eligibility with a free consultation and a prescriber will review your case the same day.