How Wegovy and Mounjaro are different — and why that matters for your decision

Two medicines, two different biological targets

The comparison between Wegovy and Mounjaro begins with biology. Semaglutide (Wegovy) is a GLP-1 receptor agonist: it mimics the gut hormone GLP-1, slowing gastric emptying, suppressing appetite and improving blood-sugar regulation. Tirzepatide (Mounjaro) does all of that, and also activates GIP receptors, making it the only dual-agonist weight-loss medicine licensed in the UK. Whether activating two pathways instead of one translates to a meaningful clinical advantage for any individual person is exactly the kind of question a prescriber needs to explore with you.

This distinction is worth understanding clearly because the names change depending on where you look. Mounjaro is Eli Lilly's brand for tirzepatide; Wegovy is Novo Nordisk's brand for semaglutide as a weight-loss injection. You may also come across a more detailed look at how tirzepatide and semaglutide differ at the molecular level if you want to go further into the science. For now, the short version: same general class of medicine, meaningfully different mechanisms.

One medicine that often comes up in these conversations is Ozempic, also semaglutide, but licensed for type 2 diabetes rather than weight management; the differences between Mounjaro and Ozempic are worth reading about separately if that's adding to your confusion.

What the clinical trials showed, and how to read the numbers fairly

Trial results are often the first thing people look up, and they're worth reading carefully. In SURMOUNT-1, participants taking tirzepatide at the highest dose achieved an average body-weight reduction of around 20–21% over 72 weeks, according to the published results in the New England Journal of Medicine. The STEP 1 trial, also published in the New England Journal of Medicine, found that semaglutide 2.4mg produced an average reduction of around 15% over 68 weeks.

The SURMOUNT-5 head-to-head trial (the first direct comparison, published in the NEJM in 2025) found that tirzepatide produced greater average weight loss than semaglutide 2.4mg over 72 weeks in adults with obesity and no diabetes. NICE's appraisal of tirzepatide also noted that indirect comparisons favour it. However, Novo Nordisk has since received UK approval for a higher 7.2mg maintenance dose of semaglutide, with trial data reporting roughly 20.7% average weight loss, narrowing the gap considerably. A clear side-by-side breakdown of Wegovy vs Mounjaro covers these figures in more detail.

The table below summarises the headline differences.

Sources: NICE TA1026 (tirzepatide); NICE TA875 (semaglutide); NEJM SURMOUNT-1 and STEP 1 trials. Numbers are trial averages; individual results vary.

Side effects, tolerability and everyday practicalities

The two medicines share a broadly similar side-effect profile because both slow gastric emptying. Nausea, loose stools, constipation, reflux, burping and fatigue are the most commonly reported effects for both, typically most noticeable in the early weeks or after a dose increase, then settling. There is not yet enough head-to-head data to say definitively that one is better tolerated than the other; what matters more is how your body responds, which varies between people.

Practically, the formats differ in ways some people care about. Mounjaro's KwikPen contains four doses, you use it weekly for a month. Wegovy comes as a weekly pen, and the recently approved 7.2mg maintenance option uses a dedicated single-dose pen with an auto-dosing mechanism. Neither requires refrigeration on the day of use, though both should be stored in the fridge between 2–8°C; the exact room-temperature window for each is set out in the Patient Information Leaflet, and that's where you should check it rather than taking a number from a website.

The practical comparison of Wegovy and Mounjaro goes into more depth on day-to-day use. For the cost difference between the two, there's a dedicated page that covers what private pricing typically looks like across providers.

Which one is right for you? That's what our prescribers are here to work out

If you've reached this page feeling like you've done your research but still aren't sure, that's completely understandable. These are clinically meaningful differences, and neither number nor mechanism alone tells you which medicine suits your health history, your other medicines, or your preferences. The contraception interaction is a good example: women taking oral contraceptives starting tirzepatide are advised to add a non-oral method for the first four weeks of treatment and for four weeks after each dose increase, because absorption of the pill may be reduced; NHS guidance does not flag the same concern for semaglutide. That kind of practical detail only surfaces properly in a clinical conversation.

Our prescribers review every consultation personally, the same day it's submitted. There's no algorithm making the call. You can read more about how tirzepatide compares with semaglutide across a broader set of clinical factors, or look at the treatment options we offer before starting your consultation. Which medicine suits you is a clinical decision our prescribers make with you, not something any comparison article can settle.

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