Mounjaro vs Wegovy for weight loss: what's the difference?

How does each medicine actually work, and does the mechanism matter?

Wegovy contains semaglutide, a GLP-1 receptor agonist. It mimics the gut hormone GLP-1, slowing gastric emptying and reducing appetite, the same pathway used by several diabetes medicines. It is given as a once-weekly subcutaneous injection, starting at 0.25mg and titrating up, with 2.4mg as the established maintenance dose and a newer 7.2mg option now approved by the MHRA. The NHS medicines page for semaglutide gives a clear patient-level overview of how it works.

Mounjaro contains tirzepatide and does something distinct: it activates both the GIP and GLP-1 receptors. Whether that dual action is what drives its trial results is still being studied, but it is the only medicine in this class with a dual-agonist mechanism licensed in the UK. It is also a once-weekly injection, starting at 2.5mg (a tolerability dose whose job is to settle your system before the first step up) with doses available up to 15mg. You can read more about it on our Wegovy overview page, and on the separate Mounjaro vs semaglutide page if the mechanism difference is the thing you're trying to pin down.

For practical purposes both are once-weekly injections with a slow titration designed to reduce side effects. The mechanism matters most when your prescriber is weighing up which is the better clinical fit for you specifically.

What do the trial results actually show?

Numbers are the part people want first, so here they are, with the context they need to be useful.

In SURMOUNT-1 (2,539 adults with obesity but without diabetes, 72 weeks), tirzepatide at 15mg produced an average body-weight reduction of around 20–21%, with some analyses reaching approximately 22.5%. That trial was published in the New England Journal of Medicine and underpins NICE's recommendation of tirzepatide (TA1026). In STEP 1 (68 weeks, semaglutide 2.4mg), average weight reduction was around 15%.

The more direct comparison came from SURMOUNT-5, an open-label head-to-head trial published in the New England Journal of Medicine in 2025. Over 72 weeks in 751 adults with obesity and no diabetes, tirzepatide produced greater average weight loss than semaglutide 2.4mg. If you are trying to decide between the two, our Mounjaro vs Wegovy page walks through how those results translate to a practical choice. NICE's committee discussion for TA1026 noted that indirect comparisons favour tirzepatide, while acknowledging that higher-dose semaglutide (the 7.2mg pen approved by the MHRA in April 2026) narrows that gap, with trials reporting around 20.7% average loss over 72 weeks at that dose.

Trial averages are population figures. Your result depends on your starting point, how your body responds, the dose you reach and whether you make the lifestyle changes alongside treatment. Neither medicine guarantees a specific outcome.

Are there differences in side effects or who can use them?

Both medicines share a broadly similar side-effect profile, dominated by gastrointestinal symptoms: nausea is the most reported, followed by changes in bowel habit, indigestion, fatigue and headache. These are most noticeable early on or after a dose increase and typically ease as your body adjusts. The NHS patient information for tirzepatide lists them plainly. On both medicines, the MHRA has highlighted acute pancreatitis as an infrequent but serious risk, severe, persistent stomach pain that radiates to the back warrants prompt medical attention.

Contraception is one area where Mounjaro requires a specific step that Wegovy does not: women taking oral contraceptive pills should add a non-oral method for the first four weeks of tirzepatide treatment and for four weeks after any dose increase, because tirzepatide may reduce pill absorption. NHS England's guidance on weight-management injections covers this, and it is something our prescribers discuss with every relevant patient at consultation.

Neither medicine is recommended during pregnancy, breastfeeding or for those trying to conceive, and neither is licensed for under-18s. For anyone deciding between these two treatments, our Saxenda or Wegovy for weight loss page also explores how injectable options compare more broadly, which can be useful context when you are working out which medicine is the right starting point. For anyone who wants to see how Wegovy sits alongside other semaglutide products such as Ozempic, the Wegovy vs Ozempic page explains the distinction, Ozempic is licensed for type 2 diabetes, not weight loss, and should not be treated as interchangeable.

Which one can I get, and through which route?

Both are prescription-only medicines. The legal route to either is a clinical assessment followed by a prescription issued by a registered prescriber, there is no shortcut through that step, and anyone offering these medicines without one is not a safe or legitimate source.

On the NHS, access to tirzepatide follows the phased rollout under NICE TA1026: as of June 2025, the first cohort (BMI ≥40 plus four or more qualifying conditions) became eligible through primary care. A second cohort opened in June 2026. Wegovy on the NHS remains tied to specialist weight-management services under TA875, with widely reported waiting lists. Our Ozempic vs semaglutide for weight loss page is worth a read if you want to understand how the different semaglutide products fit into the broader private and NHS landscape before making a decision. The weight loss treatments page covers what is available privately through nume when NHS routes are not yet open to you.

Through a regulated online pharmacy like nume, you can access either medicine privately without a referral. Our prescribers review every consultation personally, a real clinician reads your answers, not a queue-management system. If you're weighing up the two medicines and want a clinical view rather than a comparison table, that is exactly the conversation to have at your free consultation. Which medicine suits you is a decision our prescribers make with you, not one a page like this can make for you.