Mounjaro®
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Start journey Learn moreHow the trial evidence and UK licensing shape a semaglutide-to-tirzepatide switch
What the SURMOUNT-5 head-to-head trial actually found
For a long time, comparing tirzepatide and semaglutide for weight loss meant triangulating across separate studies run in different populations. That changed with SURMOUNT-5, published in the New England Journal of Medicine in 2025, which placed both medicines in the same trial for the first time. Over 72 weeks, in 751 adults living with obesity and without type 2 diabetes, tirzepatide produced a statistically greater average reduction in body weight than semaglutide 2.4mg. The gap was clinically meaningful, not just statistical noise.
It is worth being specific about what that means for someone already on semaglutide. The SURMOUNT-1 trial reported average weight loss of around 20 to 21 percent at tirzepatide's highest dose over the same 72-week period, while the STEP 1 trial found roughly 15 percent average loss at semaglutide 2.4mg, the then-maximum dose. Higher-dose semaglutide at 7.2mg is now licensed in the UK and narrows that gap, so the comparison is no longer as straightforward as it was two years ago. A prescriber can help you weigh where you currently sit on semaglutide against what tirzepatide's dose ladder might realistically offer. Our Wegovy versus Mounjaro comparison sets out those numbers side by side with their sources.
The decision to switch is never simply about trial averages. Individual response varies, tolerability differs between people, and the prescriber will factor in your current dose, how long you have been on treatment, and any side effects you have already experienced.
How the two medicines differ, and why that matters when switching
Semaglutide (Wegovy) activates GLP-1 receptors, which suppress appetite and slow gastric emptying. Tirzepatide (Mounjaro) does that too, and also activates GIP receptors, a second gut-hormone pathway involved in insulin secretion and fat metabolism. This dual action is why tirzepatide is sometimes described as a different class rather than a stronger version of semaglutide.
In practical terms, that distinction matters for a switch. The side-effect profiles overlap substantially: nausea, constipation, diarrhoea and reflux are reported with both, most commonly at dose increases and tending to settle within a week or two. But the intensity and timing can differ. Some people who found semaglutide's gastrointestinal effects difficult report a different experience on tirzepatide, and the reverse is also true. There is no reliable way to predict this in advance, which is why close monitoring during the transition period is standard practice.
Tirzepatide also carries a specific note about oral contraceptives: for the first four weeks of treatment and for four weeks after each dose increase, women taking the pill are advised to add a non-oral method of contraception, because tirzepatide may reduce absorption. NHS guidance notes no equivalent concern has been identified for semaglutide. If this applies to you, it is worth raising at your consultation. More on what to expect during the transition is covered in our dedicated switching guide.
A side-by-side look at the key facts
| Semaglutide (Wegovy injection) | Tirzepatide (Mounjaro) | |
|---|---|---|
| Mechanism | GLP-1 receptor agonist | Dual GIP and GLP-1 receptor agonist |
| UK licence (weight management) | BMI ≥30, or ≥27 with a weight-related condition | BMI ≥30, or ≥27 with ≥1 weight-related condition |
| Average weight loss (pivotal trial) | ~15% over 68 weeks at 2.4mg (STEP 1, NEJM) | ~20–21% over 72 weeks at 15mg (SURMOUNT-1, NEJM) |
| Head-to-head evidence | Greater average loss seen with tirzepatide (SURMOUNT-5, NEJM 2025) | Greater average loss seen with tirzepatide (SURMOUNT-5, NEJM 2025) |
| Oral contraceptive note | No specific absorption interaction identified | Add barrier method for 4 weeks at treatment start and after each dose increase |
| NICE recommendation | TA875: max 2 years, specialist weight management service | TA1026: BMI ≥35 + ≥1 comorbidity (NHS route); private criteria per SmPC |
Sources: STEP 1, New England Journal of Medicine; SURMOUNT-1, New England Journal of Medicine; NICE TA1026.
How a switch works through a regulated UK pharmacy
If you're considering this change, you may already be feeling a little stuck, plateau, side effects that have not settled, or simply curiosity about whether more is possible. That is a reasonable place to be, and it is exactly the kind of question our prescribers work through every week.
The process at nume starts with a free consultation. A GPhC-registered prescriber reviews your current treatment, dose history, weight trajectory and any relevant medical history before making a clinical judgement. Transfer patients are asked to provide evidence of their existing prescription, and any proposed dose increases require clinical justification. There are no shortcuts here because a switch between two prescription medicines is a clinical decision, not a product upgrade. If you are currently on Mounjaro and wondering whether to change, our guide on swapping from Mounjaro to Wegovy walks through the key considerations in detail. Equally, if you are on semaglutide and considering tirzepatide, our page on moving from semaglutide to tirzepatide covers what that process typically involves. You can also read more about our clinical team or explore the full range of treatments we offer before you start. The cost difference between Mounjaro and Wegovy is also worth understanding before your consultation, so you can have an informed conversation about what works for you practically as well as clinically.
Which medicine suits you is a clinical decision our prescribers make with you, taking into account your history, preferences and how your body has responded so far. Start your free consultation to begin that conversation.
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