Mounjaro®
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Start journey Learn moreHow semaglutide and tirzepatide compare across the things that matter most
Do the two medicines actually work differently, or is it just marketing?
This is probably the most common misconception our prescribers come across, the idea that these two medicines are essentially the same thing with different branding. They are not. Semaglutide (sold for weight management as Wegovy) activates the GLP-1 receptor, which signals fullness, slows how quickly the stomach empties and reduces appetite. Tirzepatide (sold as Mounjaro) activates two receptors: GLP-1 and GIP. That second pathway adds a distinct metabolic effect, and the clinical evidence suggests it produces meaningfully greater weight loss on average. So the mechanism is genuinely different, not just a commercial distinction.
Both are once-weekly subcutaneous injections. Both are licensed in the UK for adults with a BMI of 30 or above, or 27 or above alongside a weight-related condition such as high blood pressure or type 2 diabetes. Both carry a Black Triangle (▼) status, meaning the MHRA collects additional safety data on them. The NHS medicines page for tirzepatide and its semaglutide equivalent are the clearest public summaries of what each medicine is and how it works.
What does the head-to-head trial evidence actually show?
For a long time, comparisons between semaglutide and tirzepatide relied on separate studies in different populations, useful, but not definitive. SURMOUNT-5 changed that. This was a direct head-to-head trial (72 weeks, 751 adults with obesity and no diabetes) comparing tirzepatide against semaglutide 2.4mg, and tirzepatide produced greater average weight reduction. The Wegovy versus Mounjaro evidence page goes through those numbers in detail.
For context, SURMOUNT-1 showed tirzepatide at its highest dose achieving around 20–21% average body-weight reduction over 72 weeks, cited in the SURMOUNT-1 paper in the New England Journal of Medicine. The STEP 1 trial for semaglutide 2.4mg reported roughly 15% over 68 weeks. These were separate studies, so a direct arithmetic comparison has limits, but the gap is consistent with SURMOUNT-5's findings.
One thing worth noting: Wegovy now also comes in a 7.2mg dose, approved by the MHRA in January 2026 and available as a dedicated single-dose pen from April 2026. Early data suggest average weight loss at 7.2mg approaches tirzepatide's results more closely, which narrows the gap. This is a developing picture.
| Factor | Semaglutide (Wegovy) | Tirzepatide (Mounjaro) |
|---|---|---|
| Mechanism | GLP-1 receptor agonist | Dual GIP and GLP-1 receptor agonist |
| Average weight loss (pivotal trial) | ~15% at 2.4mg (STEP 1, 68 weeks) | ~20–21% at 15mg (SURMOUNT-1, 72 weeks) |
| Head-to-head result (SURMOUNT-5) | Comparator | Greater average loss vs semaglutide 2.4mg |
| UK licence (weight management) | BMI ≥30, or ≥27 with condition | BMI ≥30, or ≥27 with condition |
| NICE recommendation | TA875 (specialist services, max 2 years | TA1026) BMI ≥35 with ≥1 comorbidity |
Are the side effects of semaglutide and tirzepatide the same?
Broadly, yes, both medicines share a GI-led side-effect profile. Nausea is the most frequently reported effect, particularly after starting or after a dose step-up. Vomiting, loose stools, constipation, reflux and fatigue all appear in the trial data for both. These effects tend to be most noticeable in the first days after a new dose and ease for most people within a couple of weeks as the body adjusts.
Serious side effects are uncommon but exist for both medicines. Acute pancreatitis is a known risk; in January 2026 the MHRA issued a Drug Safety Update specifically highlighting this for GLP-1 medicines. Severe stomach pain that spreads to the back, with or without vomiting, is the signal to get urgent medical attention. You can report any suspected side effect using the MHRA Yellow Card scheme.
On the contraception point (worth flagging because it surprises people) tirzepatide can reduce absorption of the oral contraceptive pill. NHS guidance recommends adding a non-oral method for the first four weeks of tirzepatide treatment and for four weeks after each dose increase. There is no equivalent concern documented for semaglutide. If you take the pill, this practical difference between the two medicines is worth discussing with a prescriber.
Which one can you actually get, and through which route?
Both Mounjaro and Wegovy are available privately through a GPhC-registered online pharmacy following a clinical assessment. Neither requires a GP referral in the private setting. Ozempic, incidentally, is also semaglutide but licensed for type 2 diabetes rather than weight management, it is not a weight-loss medicine under its UK licence, and that distinction matters.
On the NHS, the two medicines follow different pathways. NICE's appraisal of tirzepatide (TA1026) recommends it for adults with a BMI of 35 or above alongside at least one weight-related comorbidity, through a phased rollout tied to NHS commissioning. Semaglutide (Wegovy) was recommended under TA875 but only within specialist weight management services and for a maximum of two years. Waiting lists for both NHS routes are long in most areas.
For people who do not meet the current NHS thresholds or prefer not to wait, private treatment through a regulated pharmacy is the practical alternative. At nume, a prescriber (not an automated tool) reviews every consultation the same day. How the two medicines compare on cost is a separate question worth reading before you decide. When you're ready, speak to our prescribers and they'll help you work out which treatment fits your situation.
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