Switching from Wegovy to Zepbound: what the evidence says for UK patients

What the head-to-head trial data actually shows

The most direct evidence comes from SURMOUNT-5, published in the New England Journal of Medicine in 2025. In that open-label trial (751 adults with obesity and no type 2 diabetes, followed for 72 weeks) tirzepatide produced greater average weight reduction than semaglutide 2.4 mg. SURMOUNT-1 had already shown tirzepatide at 15 mg achieving around 20 to 21 percent average body-weight reduction in clinical trials. The STEP 1 trial, which studied semaglutide 2.4 mg over 68 weeks, reported roughly 15 percent average loss in that setting.

That gap narrowed somewhat with the arrival of Wegovy 7.2 mg, approved by the MHRA in early 2026, which reported around 20.7 percent average weight loss over 72 weeks. If you want to dig into how Wegovy and Zepbound compare specifically on weight loss outcomes, including what the trial numbers mean in practice, that page covers the detail. What the trials cannot tell you is how your body specifically will respond to either, individual variation is real, and trial averages describe populations, not individuals.

The mechanistic difference matters here. Mounjaro activates two receptors, GIP and GLP-1; Wegovy activates one. Whether that dual action produces a meaningfully different clinical outcome for a given patient is precisely what a prescriber weighs up. You can read how these two medicines compare across a wider set of factors if you want the fuller picture before a consultation.

Zepbound and Mounjaro are the same medicine, but the geography matters

A question our prescribers hear most weeks is whether patients can source Zepbound from abroad because they have seen it mentioned online or because a US-based contact is using it. The honest answer is straightforward: Zepbound is tirzepatide approved by the US FDA, sold under that name by Eli Lilly in the United States. In the UK, tirzepatide is licensed and dispensed as Mounjaro. The active molecule is the same; the regulatory approval, supply chain, and labelling are not.

UK law requires that prescription-only medicines be supplied through a pharmacy registered with the GPhC and prescribers regulated in the UK. Importing Zepbound for personal use sits outside that framework. Any seller offering it as a UK product, or without a valid UK prescription, is operating outside the law. The MHRA has warned explicitly about counterfeit and illegally traded weight-loss pens; anyone considering buying from unlicensed sources can report suspected sellers at the MHRA's Yellow Card site.

If you are already on Wegovy and want to know whether a switch to tirzepatide (Mounjaro) makes clinical sense, that is a legitimate, answerable question, just one for a prescriber rather than an online search. Our page on switching from Wegovy to Zepbound explains what that process involves and why the UK context changes the practical steps. The comparison of Zepbound versus Wegovy as concepts explains the US context in more depth if that is where your curiosity started.

What a prescriber looks at before approving any switch

Switching from one GLP-1 medicine to another is not simply a matter of stopping one pen and starting another. A prescriber will want to know how long you have been on Wegovy, what dose you reached, how much weight you lost and over what period, what side effects you experienced, and whether any of those effects are likely to shift on a different medicine. NICE's guidance on tirzepatide (TA1026) sets out the clinical framework for its use in adults with obesity, including the threshold for reviewing continuation if weight loss falls below five percent at the highest tolerated dose after six months, a benchmark that is equally relevant when thinking about switching.

There is also the question of eligibility. Tirzepatide is licensed privately for adults with a BMI of 30 or above, or 27 or above alongside at least one weight-related condition. Wegovy carries broadly comparable private-route criteria. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance. Neither medicine is suitable during pregnancy, while breastfeeding, or if you are actively trying to conceive; suitability for under-18s is not established. A prescriber also needs to know about other medicines you take, particularly oral contraceptives, for tirzepatide there is guidance around adding a non-oral method of contraception for the first four weeks of treatment and for four weeks after each dose increase.

If you have previously switched in the other direction, the considerations for switching from Mounjaro to Wegovy follow a similar logic. The clinical review comes first, in either direction.

What is actually available for weight management in the UK

For UK patients, the two licensed injectable weight-management medicines are Wegovy (semaglutide, by Novo Nordisk) and Mounjaro (tirzepatide, by Eli Lilly). Since June 2026 there is also Wegovy in tablet form (oral semaglutide, the first oral GLP-1 medicine licensed for weight management in the UK) for those who prefer not to inject. Patients who are weighing up the two injectables often ask whether Zepbound is better than Wegovy, and that page works through the evidence to help you form a view before speaking to a prescriber. You can also explore all treatment options before you decide what to discuss with a prescriber.

The table below summarises the key factual comparison between Wegovy and Mounjaro, the two injectable options a UK patient would actually be choosing between.

Which of these suits your situation is a clinical decision our prescribers make with you, based on your health history, any medicines you take, your preferences, and what you have already tried. Speak to our prescribers to begin that conversation.