Mounjaro®
Starting from £179.99/mo
Start journey Learn moreThe mechanism gap that explains the trial results
The myth worth correcting first: tirzepatide is not just a stronger GLP-1
A lot of people arrive at this question assuming tirzepatide is simply a more powerful version of semaglutide or liraglutide. It isn't. It belongs to a different pharmacological class entirely. Pure GLP-1 receptor agonists, such as semaglutide (the active ingredient in Wegovy) and liraglutide (Saxenda), act on one receptor pathway involved in appetite signalling and blood-sugar regulation. Tirzepatide, sold in the UK as Mounjaro, acts on two: the GLP-1 receptor and the GIP receptor simultaneously. GIP is a separate gut hormone that has its own effects on fat metabolism and insulin sensitivity, and activating both pathways together appears to produce a different overall result than activating either alone.
This dual action is why the NICE appraisal of tirzepatide (TA1026) notes indirect comparisons favouring tirzepatide over semaglutide, and why the subsequent SURMOUNT-5 head-to-head trial confirmed a greater average weight reduction with tirzepatide than with semaglutide 2.4mg over 72 weeks. If you want a closer look at how the two relate, our page explaining what the difference between Mounjaro and tirzepatide actually is covers that in full. Tirzepatide is the only dual GIP/GLP-1 agonist licensed for weight management in the UK, and if you are also weighing up retatrutide as an option, our page on the difference between reta and Mounjaro sets out clearly where those two medicines stand relative to each other. Worth knowing before you compare prices or trial numbers between these medicines.
What GLP-1-only medicines actually do (and where they sit in the UK)
When people ask about GLP-1 medicines, they usually mean one of three: semaglutide injection (Wegovy, licensed for weight management), liraglutide injection (Saxenda, also licensed for weight management), or (and this matters) semaglutide under a different brand name used for type 2 diabetes. Ozempic contains semaglutide and is licensed in the UK for type 2 diabetes, not weight loss. The NHS medicines page for semaglutide sets this out clearly. Rybelsus is oral semaglutide, again licensed for diabetes, not weight management. Using these medicines for weight loss when they haven't been prescribed for it is not appropriate; it also contributes to the supply shortages that affect patients who need them for diabetes.
Wegovy (semaglutide 2.4mg injection, titrated from 0.25mg) is the legitimate weight-management version. In the STEP 1 trial it produced around 15% average weight loss over 68 weeks. A higher 7.2mg dose has since been approved by the MHRA. Liraglutide (Saxenda) is also licensed but follows a daily injection schedule rather than weekly. If you're interested in how liraglutide compares specifically with tirzepatide, there's a fuller breakdown on the liraglutide vs tirzepatide page.
Side-by-side: GLP-1-only vs dual GIP/GLP-1 in numbers
The table below compares the two licensed weekly injections most directly relevant to this question. Both are prescription-only medicines; the figures come from their respective registration trials and the NICE appraisals referenced above.
| Feature | Semaglutide 2.4mg (Wegovy injection) | Tirzepatide up to 15mg (Mounjaro) |
|---|---|---|
| Receptor target | GLP-1 only | GLP-1 + GIP (dual agonist) |
| Average weight loss (licensed trial) | ~15% over 68 weeks (STEP 1) | ~20–21% over 72 weeks (SURMOUNT-1) |
| Injection frequency | Once weekly | Once weekly |
| UK licence: weight management | Yes (Wegovy) | Yes (Mounjaro) |
| NICE recommended | Yes (TA875, within specialist services, max 2 years) | Yes (TA1026, from Dec 2024) |
| Head-to-head evidence | SURMOUNT-5 (2025): greater loss with tirzepatide | SURMOUNT-5 (2025): greater loss with tirzepatide |
One practical habit: before you read any headline comparison figure, check which dose it refers to. The 15% figure for semaglutide is from its 2.4mg maintenance dose. Wegovy's newer 7.2mg dose, approved by the MHRA in 2026, reported around 20.7% average weight loss, which narrows the gap with tirzepatide considerably.
Which one fits you depends on the full clinical picture
Neither medicine is universally superior for every individual. Side-effect profiles overlap heavily (nausea, changes in bowel habit, and reduced appetite are common with both) but there are meaningful differences in how each medicine behaves at the receptor level, how they interact with oral contraceptives, and the NHS access pathways available to you. Tirzepatide has a specific interaction note for women on the pill: a non-oral contraceptive method is recommended for the first four weeks of treatment and for four weeks after each dose increase, because GI changes can reduce absorption of oral hormones. The NHS England page on weight-management injections covers this interaction clearly.
Eligibility also differs. The NICE threshold for Mounjaro on the NHS currently requires a BMI of at least 35 plus multiple comorbidities, with the phased rollout ongoing. Private eligibility via a licensed pharmacy is broader: BMI of 30 or above, or 27-plus with a relevant weight-related condition. A prescriber looks at the full picture, not just the numbers. You can explore what that assessment involves on the weight-loss treatments page, or read about Mounjaro's cost in the UK if that's the next question on your list. For comparison with medicines that are still in development or not yet widely licensed, our page on how Mounjaro and retatrutide differ covers where those candidates stand. Which medicine suits you is a clinical decision our prescribers make with you, based on your health history, your goals, and what the evidence supports.
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