Switching from Wegovy to Zepbound — what UK patients need to know

What the head-to-head trial found, and why it matters for anyone considering a switch

The most important piece of evidence here is SURMOUNT-5, published in the New England Journal of Medicine in 2025. It was an open-label trial involving 751 adults living with obesity, none of whom had type 2 diabetes. Participants received either tirzepatide or semaglutide 2.4mg weekly for 72 weeks. Tirzepatide produced greater average weight reduction across all doses tested. That is a meaningful finding because previous comparisons were indirect (constructed by placing two separate trials side by side) whereas SURMOUNT-5 randomised people to one medicine or the other under identical conditions.

NICE cited this and similar indirect evidence in its appraisal of tirzepatide (TA1026, published December 2024), noting the direction of effect consistently favoured tirzepatide. The STEP 1 trial, which established semaglutide 2.4mg's licence, reported roughly 15% average body-weight reduction over 68 weeks. SURMOUNT-1 reported roughly 20–21% at the 15mg tirzepatide dose over 72 weeks. Headline figures from different trials cannot be added directly (population differences matter) but the SURMOUNT-5 head-to-head narrows the interpretive gap considerably.

For someone who has been on Wegovy and wants to know whether switching from Mounjaro to Wegovy, or the reverse, might produce better results, this evidence is relevant. It does not guarantee anything for any individual. Biology, dose history, tolerability and other health factors all shape what happens in practice. That is precisely why a prescriber rather than a website makes this call.

Sources: SURMOUNT-1, New England Journal of Medicine; STEP 1, New England Journal of Medicine; NICE TA1026 recommendations.

Why Zepbound specifically is not an option in the UK, and what to do instead

Zepbound is tirzepatide as approved and branded by Eli Lilly for the US market. It does not hold a UK marketing authorisation and cannot legally be supplied by any UK pharmacy, online or otherwise. Sellers advertising Zepbound to UK buyers are offering a product outside the licensed supply chain. That matters more than it might sound: the MHRA has warned repeatedly about counterfeit and unlicensed weight-loss medicines circulating online, with fake pens containing wholly different substances in some seizures. You can report any suspect seller at the MHRA Yellow Card site.

For people researching switching from Zepbound to Wegovy, or the reverse (often because they are relocating from the US or have read American coverage of tirzepatide) the practical UK answer is Mounjaro. It is exactly the same molecule, the same pre-filled KwikPen design, the same dose strengths from 2.5mg to 15mg. If you were established on a particular Zepbound dose in the US, your prescriber can discuss whether a comparable Mounjaro dose is appropriate, though any transition requires a clinical review rather than a straight substitution. Our page on whether switching from Wegovy to Zepbound is possible in the UK covers the regulatory detail more fully.

If you are currently on Wegovy and considering a move to Mounjaro instead, that is a different and legitimate clinical question. The evidence above is part of what a prescriber would weigh. So would your current dose, any side effects you have had, your other health conditions and your treatment goals.

Side-effect profile differences, what changes when you switch between these two medicines

Both medicines share a broadly similar tolerability picture because both slow gastric emptying and reduce appetite through gut-hormone pathways. Nausea, loose stools, constipation, indigestion and fatigue are common in the early weeks with either treatment, typically easing after the first few weeks or after a dose step. The NHS patient-information pages for tirzepatide and semaglutide set these out side by side in plain English if you want to compare them directly.

One difference worth knowing about before switching: tirzepatide activates both the GIP and GLP-1 receptor, while semaglutide activates only the GLP-1 pathway. Some people who found GI side effects difficult on semaglutide report a different profile on tirzepatide, though individual responses vary and no blanket prediction is reliable. More practically, if you switch, you would normally restart at the lowest titration dose rather than carrying your current dose across, your system needs time to adjust to a new molecule. A prescriber decides the starting point, not a comparison article.

There is also an important note for anyone switching and taking an oral contraceptive. Tirzepatide can reduce absorption of the pill during the first four weeks of treatment and for four weeks after each dose increase; adding a non-oral method during those windows is the current MHRA-backed advice. The equivalent concern has not been identified for semaglutide. If this applies to you, it is worth raising in your consultation before any switch begins. Our comparison of Wegovy and Mounjaro covers this and other practical differences in more detail.

How the cost picture compares, and how to think about it

Pricing moved significantly in late 2025. Eli Lilly raised Mounjaro's UK list price from 1 September 2025; reported market prices since then have ranged roughly £149–£375 per month depending on dose and provider. Wegovy sits in a broadly comparable range. If you are weighing up a switch partly on cost, the honest framing is that neither is cheap, and the gap between them varies more by provider and dose than by medicine. Our page on the price difference between Mounjaro and Wegovy goes into this properly.

At nume, one transparent price covers the consultation, the prescription, the treatment and free next-working-day tracked delivery, no hidden fees, no subscription. Orders placed by 12pm Monday to Friday, once approved, go out the same day. If you happen to be placing an order around payday or before a bank holiday, it is worth factoring in that next-working-day means exactly that, so a Friday order might not arrive until Tuesday. We are not the cheapest option and we have not tried to be. What that price buys is a real prescriber reading your notes, not software, and aftercare available seven days a week.

Which of these medicines suits your situation is a clinical decision, and our prescribers make it with you rather than for you. The free consultation is the right place to start that conversation. You might also find it useful to read our fuller look at how Zepbound and Wegovy compare from a US-versus-UK licensing perspective, or our breakdown of weight-loss outcomes between the two molecules.