Mounjaro®
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Start journey Learn moreThe Clinical Evidence: What the Trials and UK Guidance Tell Us
Step 1 — Understanding the receptor difference
GLP-1 receptor agonists mimic glucagon-like peptide-1, a gut hormone that slows gastric emptying, dampens appetite and helps regulate blood sugar. Semaglutide (sold as Wegovy for weight management and Ozempic for type 2 diabetes) works on this pathway alone. Ozempic, it is worth stating plainly, carries a UK licence for diabetes only, not for weight loss.
Tirzepatide, sold in the UK as Mounjaro, activates both GLP-1 and GIP receptors simultaneously. GIP (glucose-dependent insulinotropic polypeptide) plays its own role in fat storage and energy balance. Recruiting both pathways appears to produce additive effects on appetite reduction that a single-agonist medicine cannot fully replicate. The NHS medicines page for tirzepatide describes this dual mechanism clearly, and it is the scientific basis for the outcome differences seen in trials.
This is not a minor technical distinction. The way a medicine binds to receptors shapes its side-effect profile, its titration schedule and, ultimately, how much weight a given patient is likely to lose on it. Starting dose, pace of increase and maximum strength all flow from this biology.
Step 2, What the head-to-head evidence shows
For years, comparisons between tirzepatide and semaglutide relied on indirect analysis of separate trials. That changed with SURMOUNT-5, an open-label trial published in the New England Journal of Medicine in 2025, which randomised 751 adults with obesity and no diabetes to tirzepatide or semaglutide 2.4mg for 72 weeks. Tirzepatide produced significantly greater average body-weight reduction. NICE's appraisal of tirzepatide (TA1026) acknowledges the indirect comparisons favoured tirzepatide before SURMOUNT-5 landed; the head-to-head result confirmed the direction of that signal.
SURMOUNT-1 reported average weight loss of around 20–21% at the 15mg tirzepatide dose over 72 weeks among participants without diabetes, in a study of 2,539 adults. The STEP 1 trial for semaglutide 2.4mg reported approximately 15% average reduction at 68 weeks. Wegovy's newer 7.2mg dose (approved by the MHRA in January 2026) has narrowed that gap somewhat, with trial data suggesting around 20.7% average loss over 72 weeks. These figures all come from specific trial populations and conditions; individual results vary, and none of them should be read as a personal prediction.
The honest takeaway: tirzepatide consistently produces larger average reductions in the available data, but a 15% average loss is still clinically meaningful, and the medicine that suits someone best is rarely decided by the headline trial number alone.
Step 3 (Eligibility and UK licensing side by side
| Factor | Tirzepatide (Mounjaro) | Semaglutide 2.4mg (Wegovy injection) |
|---|---|---|
| Mechanism | Dual GIP + GLP-1 agonist | GLP-1 agonist |
| UK weight-loss licence | Yes) BMI ≥30, or ≥27 with a weight-related condition | Yes (BMI ≥30, or ≥27 with a weight-related condition |
| NICE recommendation | TA1026: BMI ≥35 + ≥1 comorbidity (NHS route) | TA875: BMI ≥35 + ≥1 comorbidity, specialist service, max 2 years (NHS route) |
| Average trial weight loss | ~20–21% at 15mg (SURMOUNT-1, 72 weeks) [Source: NEJM] | ~15% at 2.4mg (STEP 1, 68 weeks); ~20.7% at 7.2mg [Source: NEJM; MHRA] |
| Dosing frequency | Once weekly injection | Once weekly injection |
| Black Triangle status | Yes) additional MHRA monitoring | Yes, additional MHRA monitoring |
Lower BMI thresholds apply for some ethnic backgrounds under UK guidance, a point worth raising at your consultation. The table above covers the weight-management licence; for anyone asking about how GLP-1 medicines relate to Mounjaro specifically, that comparison goes deeper on the mechanism question.
Step 4, Practical factors that often decide the choice
Trial averages do not account for tolerability. Some people find one medicine's side-effect profile easier to manage than another's. Both tirzepatide and semaglutide share a GI-led side-effect picture (nausea, loose stools, constipation, reflux) typically most noticeable at the start or after a dose increase, often settling within a couple of weeks. The NHS semaglutide medicines page and the equivalent tirzepatide page detail these clearly.
There are also practical considerations. Women using oral contraceptives starting tirzepatide should add a non-oral method for the first four weeks of treatment and for four weeks after each dose increase, because tirzepatide may reduce pill absorption. No equivalent evidence exists for semaglutide. Anyone taking HRT should discuss whether a transdermal option is preferable with their prescriber before starting tirzepatide. These are the kinds of clinical details that shape which medicine is right for a specific person.
If you have already been taking a GLP-1 medicine and are considering moving, the practical steps involved are covered on our switching from a GLP-1 to tirzepatide page. Cost context matters too, the Mounjaro pricing guide sets out what drives the price variation you see between providers.
Which medicine suits you is not a choice an article can make. It is a clinical decision our prescribers work through with you, based on your health history, your current medicines and your individual goals. If you want to explore how the active ingredient relates to the branded product before booking, our page on tirzepatide vs Mounjaro explains the relationship, and we also have a tirzepatide vs Manjaro page for anyone who has seen that spelling used elsewhere. Check your eligibility and start a free consultation to have that conversation with a real prescriber, the same day you apply.
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