Mounjaro®
Starting from £179.99/mo
Start journey Learn moreHow Mounjaro and Wegovy compare: mechanism, evidence and eligibility
You're looking at two treatments side by side and wondering which one a prescriber would actually choose for you
That's the position a lot of people are in, and it's a reasonable one. Both Mounjaro and Wegovy sit in the GLP-1 drug class, both slow gastric emptying and reduce appetite, and both require an NHS or private prescription. The Wegovy vs Mounjaro question comes up in our prescribers' consultations constantly.
The key structural difference is the receptor mechanism. Mounjaro (tirzepatide, made by Eli Lilly) binds to both the GLP-1 and GIP receptors — making it the only dual-agonist weight-loss medicine currently licensed in the UK. Wegovy (semaglutide, made by Novo Nordisk) targets the GLP-1 receptor only. That single extra pathway is thought to explain a meaningful part of the difference in average weight loss seen across trials, though individual responses still vary considerably. If you want to understand the Mounjaro and Wegovy difference in detail, the clinical reasoning behind that receptor distinction also shapes how each medicine is titrated and what side effects tend to appear when.
Both are Black Triangle (▼) medicines under additional MHRA monitoring. Neither is suitable for use in pregnancy, during breastfeeding, or for people under 18. A prescriber reviews the full picture (other medicines, medical history, current BMI) before recommending either one.
What the trial data actually shows, and the table that puts it side by side
The headline trial figures are worth knowing, even if they are population averages rather than personal predictions. In SURMOUNT-1 (72 weeks, tirzepatide, published in the New England Journal of Medicine), participants on the 15mg dose lost around 20–21% of body weight on average. In STEP 1 (68 weeks, semaglutide 2.4mg), average weight loss was around 15%.
A direct head-to-head followed: the SURMOUNT-5 trial compared tirzepatide against semaglutide 2.4mg in adults with obesity but without diabetes over 72 weeks. Tirzepatide produced greater average weight reduction. NICE's appraisal of tirzepatide (TA1026) notes that indirect comparisons also favour tirzepatide, though it acknowledges the limitations of cross-trial comparison. Novo Nordisk has since received MHRA approval for a 7.2mg Wegovy dose, with trial data showing around 20.7% average weight loss, narrowing the gap at the highest doses.
| Feature | Mounjaro (tirzepatide) | Wegovy (semaglutide) |
|---|---|---|
| Active ingredient | Tirzepatide | Semaglutide |
| Receptor mechanism | Dual GIP + GLP-1 agonist | GLP-1 agonist only |
| Dosing schedule | 2.5mg → 5 → 7.5 → 10 → 12.5 → 15mg, stepped by prescriber | 0.25mg → 0.5 → 1.0 → 1.7 → 2.4 → 7.2mg, stepped by prescriber |
| Average weight loss (trial) | ~20–21% at 15mg (SURMOUNT-1, 72 weeks) | ~15% at 2.4mg (STEP 1, 68 weeks); ~20.7% at 7.2mg |
| NICE recommendation | TA1026: BMI ≥35 + ≥1 comorbidity (private licence: BMI ≥30 or ≥27 with condition) | TA875: BMI ≥35 + ≥1 comorbidity, specialist service, max 2 years (private licence similar) |
| MHRA monitoring status | Black Triangle (▼) | Black Triangle (▼) |
Sources: SURMOUNT-1, NEJM; NICE TA1026; NICE TA875. Trial figures are population averages, not guaranteed individual results.
Side-effect profiles, eligibility and the oral option, the practical differences
The side-effect profiles overlap considerably. Both medicines produce mainly gastrointestinal effects, nausea, constipation, loose stools, reflux and fatigue are reported with each. These tend to be most noticeable in the early weeks of treatment or after a dose step, then settle for most people. The NHS tirzepatide page and the equivalent semaglutide page carry the full lists. The key safety point that applies to both: seek urgent medical help for severe, persistent stomach pain that spreads to the back, as this can be a sign of pancreatitis.
There is one contraception-specific difference. Women using oral contraceptive pills should add a barrier method for the first four weeks of Mounjaro treatment and for four weeks after each dose increase, because the pill's absorption may be reduced. Current evidence does not show the same effect with semaglutide, though a prescriber should review all medications before starting either treatment.
Private eligibility for both medicines starts at BMI 30 or above, or BMI 27 or above alongside a weight-related condition such as high blood pressure, type 2 diabetes or obstructive sleep apnoea. Lower thresholds apply for some ethnic backgrounds under UK guidance. Separately, Novo Nordisk now offers a once-daily tablet form of semaglutide for weight management, the treatment options overview covers that alongside both injections.
If you're weighing this up close to a bank holiday or payday Monday, it's worth knowing that orders placed before 12pm on any working day are dispatched the same day once a prescriber has reviewed and approved them.
Which medicine is the better fit depends on your health history, your current medicines, your tolerance for injection vs. tablet, and the prescriber's clinical judgement. That's not a deflection, it genuinely is a decision that benefits from a real clinician's eye. If you'd like a closer look at how the two treatments compare across the details that matter most, our page on what's the difference between Wegovy and Mounjaro walks through them, or you can read more about how pricing compares between the two before you decide.
Our prescribers review every consultation personally. If you'd like their view on which treatment fits your situation, and want to understand the difference between Mounjaro and Wegovy from a clinical angle before you start, start your free consultation and they'll work through it with you.
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