Compare Mounjaro and Wegovy: what the evidence actually shows

You've been told to try one of them, but no one explained the difference

That's the situation a lot of people are in. A GP mentions Mounjaro; a friend swears by Wegovy; an article cites a trial you half-read. The names get used interchangeably online, but they are different medicines with different mechanisms, different trial programmes and (as of summer 2026) different licensing updates to keep track of.

The active ingredient in Mounjaro is tirzepatide, made by Eli Lilly. It works on two gut-hormone receptors simultaneously: GLP-1 and GIP. Wegovy's active ingredient is semaglutide, made by Novo Nordisk, and it targets GLP-1 alone. In practice, both slow gastric emptying and reduce appetite. The dual-receptor action of tirzepatide is what many researchers believe accounts for its stronger average results in trials, though individual response varies considerably. For a closer look at how semaglutide and tirzepatide differ at the molecular level, this page explores the pharmacology in more detail.

Both carry a Black Triangle (▼) designation, meaning the MHRA is collecting additional safety data. Both are licensed for adults with a BMI of 30 or above, or 27 and above with at least one weight-related condition such as high blood pressure or type 2 diabetes. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance. Neither is licensed for use in pregnancy or while breastfeeding, and neither is currently licensed for under-18s.

What the head-to-head trial actually found

For years, comparing Mounjaro and Wegovy meant looking at separate trials run at different times, against placebo, in populations that weren't identical. That changed with SURMOUNT-5, a head-to-head study published in the New England Journal of Medicine in 2025, which randomised 751 adults with obesity but without diabetes to tirzepatide or semaglutide 2.4mg for 72 weeks. Tirzepatide produced greater average weight reduction. A full breakdown of the weight-loss figures from both trials is on this page.

Wegovy's story didn't end there. The MHRA approved a higher 7.2mg maintenance dose of semaglutide in January 2026, and a dedicated single-dose 7.2mg pen on 14 April 2026, for adults with a BMI of 30 or above. Trials at 7.2mg reported around 20.7% average weight loss over 72 weeks, narrowing the gap with tirzepatide's 15mg results considerably. That development is worth factoring in when you compare Wegovy to Mounjaro across all the latest data, since the headline figures quoted in many articles predate that approval. More on how the 7.2mg dose changes the comparison is covered here.

The table below summarises the key factual differences. Numbers are drawn from published trial data and NICE appraisals.

Side effects and day-to-day experience

The side-effect profiles of the two medicines overlap heavily. Both commonly cause nausea, vomiting, diarrhoea, constipation and indigestion, particularly in the first weeks of treatment and after dose increases. These effects typically ease as the body adjusts, though the timeline varies by person. Injection-site reactions, fatigue and headache appear on both medicines' reported lists.

Because tirzepatide affects two hormone pathways, women starting Mounjaro who take the oral contraceptive pill are advised to add a non-oral method of contraception for the first four weeks of treatment and for four weeks after each dose increase, slowed gastric emptying can reduce pill absorption. The equivalent caution does not currently apply to semaglutide, according to NHS England's guidance on weight-management injections. This is a practical difference worth raising at consultation, particularly for people who rely on the pill as their primary contraception.

Storage is similar for both: refrigerated between 2–8°C, with a limited window at room temperature per each medicine's Patient Information Leaflet. In practice, many people keep their pen in the fridge door and set a weekly reminder; the specifics of that window differ between products and are set out in their respective leaflets rather than here.

For a fuller side-by-side on practical tolerability, this comparison page covers what patients commonly report. Questions about long-term use, contraindications or switching between the two are best answered by a prescriber who can see your full medical picture, which is exactly what our clinical team does. You can read about how we approach that on our clinical team page.

NHS access and the private route

Both medicines have NICE approval, but NHS access remains restricted and phased. Tirzepatide (NICE TA1026) is being rolled out in primary care in stages, currently prioritising people with a BMI of 40 or above and four or more qualifying conditions. Semaglutide under TA875 requires a specialist weight management service, a multidisciplinary care package, and carries a two-year maximum duration on the NHS. Waiting lists for those services are commonly long. Data on prescribing volumes and NHS uptake is explored separately here.

For people who don't meet the current NHS thresholds, or who don't want to wait, a private prescription from a regulated online pharmacy is the legal alternative. That means a genuine clinical consultation, identity verification and a prescriber who reviews your application personally before anything is issued. At nume, that review happens the same working day. Treatment is then dispatched for next-working-day delivery if ordered before 12pm. You can explore both options on our treatment page.

Which of these two medicines suits you is a clinical decision our prescribers make with you, based on your BMI, your health history, your other medications and your preferences. Neither medicine is universally the right choice.