Mounjaro or semaglutide for weight loss: how do they compare?

Do Mounjaro and semaglutide actually work the same way?

Not quite. Semaglutide activates a single gut-hormone receptor (GLP-1) which suppresses appetite, slows gastric emptying and signals fullness to the brain. Mounjaro (tirzepatide) activates two receptors: GLP-1 and GIP. That dual action appears to affect appetite and metabolic regulation through slightly different pathways, which may partly explain why the average weight loss seen in trials has tended to be larger with tirzepatide. The NHS medicines pages for both tirzepatide and semaglutide explain their mechanisms in plain language worth reading before your consultation.

Both are once-weekly subcutaneous injections that start at a low dose and are titrated upward by a prescriber over several months. Mounjaro begins at 2.5mg (a dose whose job is settling your system, not producing rapid change) before progressing through five further strengths up to 15mg. Wegovy follows a similar graduated ladder, reaching 2.4mg as its standard maintenance dose, with a higher 7.2mg option now also approved by the MHRA for eligible patients.

It is a common misconception that Ozempic is a weight-loss version of semaglutide. Mounjaro is not semaglutide and Ozempic is licensed for type 2 diabetes, not weight management, only Wegovy carries the UK weight-loss licence for semaglutide.

What does the trial evidence show about results?

The headline figures come from two large placebo-controlled programmes. In SURMOUNT-1, adults taking tirzepatide 15mg lost around 20–21% of body weight on average over 72 weeks. In STEP 1, adults taking semaglutide 2.4mg lost around 15% over 68 weeks. Both trials enrolled adults with obesity but without type 2 diabetes, alongside lifestyle support, and both were published in the New England Journal of Medicine.

A more direct comparison came from SURMOUNT-5, a head-to-head open-label trial published in the NEJM in 2025, which found tirzepatide produced greater average weight reduction than semaglutide 2.4mg over 72 weeks. If you want to explore what's better, Mounjaro or semaglutide, our dedicated page walks through the trial evidence, including SURMOUNT-5, in detail. The gap between the two narrows with semaglutide's newer 7.2mg dose, where trial data reported around 20.7% average loss over 72 weeks.

A summary of the key comparison points:

Numbers from named trial papers; NICE TA1026 and NICE TA875 summarise the clinical evidence basis for both recommendations.

Are the side effects and eligibility criteria different between them?

The side-effect profiles are broadly similar. Both medicines most commonly produce gastrointestinal effects (nausea in particular, sometimes vomiting or loose stools) that tend to be most noticeable during the early weeks or after a dose step-up and generally settle as the body adjusts. Neither is suitable during pregnancy, breastfeeding or while trying to conceive. Both are licensed for adults only; children and under-18s fall outside the licensed indications for either treatment.

One practical difference: for tirzepatide specifically, people taking an oral contraceptive pill should add a barrier method for the first four weeks of treatment and for four weeks after each dose increase, as absorption of the pill may be affected. NHS guidance does not flag the same concern for semaglutide. If HRT is part of your routine, it is worth discussing the form (patch, gel or oral) with a prescriber before starting tirzepatide. You can read more in our FAQs or raise this directly during your consultation.

For a fuller side-by-side look at Wegovy versus Mounjaro (including eligibility thresholds, NHS access and cost context) we have a dedicated comparison page. The cost angle specifically is covered on our tirzepatide vs semaglutide cost page.

So which should you choose?

That depends on your medical history, any medicines you already take, how you feel about injection frequency, and what your prescriber finds when they review your full picture. For some people, Wegovy is the more appropriate choice; for others, Mounjaro's dual mechanism and trial results make it the better fit. There is no blanket answer a webpage can give you responsibly.

What a page can tell you is that both are available through a legitimate private route without a referral or a waiting list, subject to clinical assessment. At nume, a named prescriber reviews your consultation the same day, not software. Once approved, your treatment leaves our GPhC-registered pharmacy the same day if you order before 12pm, arriving the next working day in a plain box with DPD tracking on your phone. Our clinical team is available seven days a week for aftercare questions once you are on treatment.

For a deeper look at the clinical question, the semaglutide or tirzepatide comparison and which is better semaglutide or tirzepatide pages explore the evidence in more detail. Ready to have the conversation? Speak to our prescribers through a free consultation and find out which option fits your situation.