Mounjaro®
Starting from £179.99/mo
Start journey Learn moreMechanism, trial data and practical differences, what the evidence shows
Two targets vs one: why the receptor difference matters
Semaglutide is a GLP-1 receptor agonist. It mimics glucagon-like peptide-1, a hormone released after eating that slows gastric emptying, suppresses appetite and nudges blood-sugar regulation. Tirzepatide does all of that, and also activates the GIP (glucose-dependent insulinotropic polypeptide) receptor. GIP is thought to enhance the appetite-suppressing effect and may influence how the body handles fat tissue differently from GLP-1 stimulation alone.
That extra receptor pathway appears to translate into measurable differences in outcomes. In the SURMOUNT-5 head-to-head trial, published in the New England Journal of Medicine in 2025, tirzepatide produced greater average weight loss than semaglutide 2.4mg over 72 weeks in adults with obesity and no diabetes. The gap narrowed with semaglutide's newer 7.2mg dose, which the MHRA approved in early 2026, but tirzepatide still holds the edge in direct comparisons at approved doses. NICE's appraisal of tirzepatide (TA1026) noted that indirect comparisons favour tirzepatide.
Neither medicine is a like-for-like substitute for the other. They share a broadly similar side-effect profile (largely gastrointestinal) but the dual mechanism means the two drugs sit in different pharmacological classes. A prescriber considers which is the better fit for your health picture, not just which produced the higher trial number.
What the trial numbers actually show (side by side
| Factor | Tirzepatide (Mounjaro) | Semaglutide (Wegovy) |
|---|---|---|
| Mechanism | Dual GIP + GLP-1 agonist | GLP-1 agonist |
| Average weight loss (pivotal trial) | ~20–21% over 72 weeks at 15mg (SURMOUNT-1, NEJM) | ~15% over 68 weeks at 2.4mg (STEP 1, NEJM) |
| Head-to-head result | Greater average loss vs semaglutide 2.4mg (SURMOUNT-5, NEJM 2025) | Narrowed gap at new 7.2mg dose |
| UK licence for weight management | Yes) BMI ≥30, or ≥27 with a weight-related condition | Yes, BMI ≥30, or ≥27 with a weight-related condition |
| NICE recommendation | TA1026 (December 2024) | TA875 (March 2023, max 2 years, specialist services) |
| Dose range | 2.5mg to 15mg, titrated by prescriber | 0.25mg to 2.4mg (or 7.2mg), titrated by prescriber |
These figures come from controlled trials under specific conditions. Individual responses vary, and trial averages are not personal predictions. The practical differences between Wegovy and Mounjaro go beyond percentages.
Eligibility, NICE recommendations and the NHS picture
Both medicines are licensed privately for adults with a BMI of 30 or above, or 27 or above alongside at least one weight-related condition such as high blood pressure, dyslipidaemia or type 2 diabetes. Lower thresholds (2.5 kg/m² less) apply for some ethnic backgrounds under UK guidance. A prescriber weighs the full picture; BMI alone is not a green light.
On the NHS, the routes differ. NICE's appraisal of semaglutide (TA875) restricts it to use within specialist weight management services for a maximum of two years, and NHS waiting lists can be long. Tirzepatide's NICE recommendation (TA1026) has its own phased NHS rollout, with GP prescribing now possible under the 2026/27 QOF contract for qualifying patients. Neither NHS route is fast or straightforward for most people right now.
Private treatment through a regulated pharmacy is available to people who meet the licensed criteria but don't want to wait. If you're mid-month and wondering whether to start before the end of the pay cycle, it's worth knowing that a same-day clinical review means you won't lose time to processing delays. Our prescribers can assess you any working day. See our treatment options for what's available and what the process involves.
Side effects and safety profile, similarities and distinctions
The two medicines share a recognisable side-effect pattern because both slow gastric emptying: nausea, loose stools, constipation, indigestion and fatigue are the most commonly reported, typically most noticeable after starting or moving up a dose, and often settling within a couple of weeks. Injection-site reactions occur with both. Serious but infrequent risks (acute pancreatitis being one flagged by the MHRA's Yellow Card scheme) apply to the GLP-1 drug class broadly and are not unique to either medicine.
One practical distinction: the MHRA advises that women taking oral contraceptives should add a barrier method for the first four weeks of tirzepatide and for four weeks after each dose increase, because tirzepatide may reduce pill absorption. There is no equivalent evidence-based warning for semaglutide. Transdermal HRT (patches or gels) is generally preferred over oral HRT while on tirzepatide for a similar reason. These are conversations to have with your prescriber before starting. For more on the relative safety profiles of semaglutide and tirzepatide, we've set out what the evidence does and doesn't show.
Ozempic (also semaglutide) is licensed for type 2 diabetes, not weight management. It is not a substitute for Wegovy, and comparing Mounjaro to Ozempic is a separate question with a different answer. If you are trying to decide whether semaglutide or tirzepatide is the right starting point for you, or you want to understand how Mounjaro, Wegovy and orlistat compare as weight loss options, we've covered both questions in detail. Which of the two weight-management medicines suits you is a clinical decision our prescribers make with you, based on your health history and what you're aiming to achieve.
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