Tirzepatide vs Mounjaro for Weight Loss: What the Evidence Shows

What the SURMOUNT trials actually found

NICE's appraisal of tirzepatide (TA1026, published December 2024) drew heavily on the SURMOUNT programme, a series of phase-3 trials involving thousands of adults with obesity. The headline figure from SURMOUNT-1 (up to around 20–21% average body-weight reduction over 72 weeks at the 15mg maintenance dose) is striking, and it comes from a rigorous randomised controlled trial. But averages obscure the spread: some participants lost considerably more, others less. What the trials do confirm is a consistent, dose-dependent effect across the 2.5mg starting dose up to 15mg, with the higher maintenance doses producing the largest sustained reductions. NICE accepted this evidence as the basis for recommending tirzepatide on the NHS for eligible adults. The dual mechanism (tirzepatide activates both GIP and GLP-1 receptors, making it the only dual-agonist weight-loss medicine licensed in the UK) is thought to underlie its efficacy profile, though head-to-head data against other agents is still accumulating. If tirzepatide is not accessible or suitable for you, it is worth exploring what can be used as a substitute for Mounjaro for weight loss and how those options compare. You can read more about what Mounjaro is and how it works if you want the mechanism explained in plain terms.

Mounjaro vs semaglutide: what the head-to-head trial showed

The more practically useful comparison for most people is Mounjaro against Wegovy (semaglutide 2.4mg), the other injectable weight-loss medicine available in the UK. The SURMOUNT-5 trial, published in the New England Journal of Medicine in 2025, ran for 72 weeks in 751 adults with obesity and no diabetes, comparing tirzepatide directly against semaglutide 2.4mg. Tirzepatide produced a greater average weight reduction. That result has since been incorporated into NICE's discussion of indirect comparisons in TA1026, which notes the evidence generally favours tirzepatide. Semaglutide's own STEP 1 trial (NEJM, 2021) showed around 15% average reduction at 2.4mg over 68 weeks, a meaningful result in its own right, and now a 7.2mg dose has also received MHRA approval, with trial data suggesting around 20.7% average loss at that higher maintenance dose. The gap narrows but does not disappear. A clean side-by-side is below.

A note on Ozempic and Rybelsus: both also contain semaglutide, but they are licensed for type 2 diabetes management, not weight loss. They are not alternatives to Wegovy or Mounjaro for people seeking weight-management treatment. The full licensed indications for Mounjaro are worth reading if you want clarity on who the medicine is formally intended for.

Side effects and tolerability: broadly similar, with individual variation

Both tirzepatide and semaglutide share a gastrointestinal side-effect profile, nausea, loose stools, constipation, indigestion, and burping are the most commonly reported effects, typically most noticeable after starting or after a dose increase, often settling within a week or two as the body adjusts. The NHS patient information for tirzepatide (NHS tirzepatide medicines page) gives a full account. Pancreatitis is a less common but serious risk: the MHRA issued a Drug Safety Update in January 2026 specifically highlighting acute pancreatitis across GLP-1 medicines, flagging severe or persistent stomach pain (especially if it spreads to the back) as a reason to seek urgent medical attention. In practice, SURMOUNT-5 found tolerability broadly comparable between the two medicines; individuals vary, and some people find one sits better with them than the other. That is one reason a prescriber's assessment matters: your medical history, current medicines and previous experience with GLP-1 treatments all shape which option is appropriate. The frequently asked questions section covers common concerns about starting treatment.

Which suits you, and how the decision gets made

The clinical evidence points to tirzepatide producing larger average weight reductions than semaglutide 2.4mg in direct comparison, and NICE's review of indirect comparisons reaches a similar conclusion. But trial averages describe populations, not individuals. Factors like BMI, existing medical conditions, other medicines (particularly oral contraceptives, tirzepatide can reduce pill absorption during the first four weeks of treatment and after each dose increase, so an additional contraceptive method is recommended during those windows), tolerance of injections versus tablets, and NHS or private access all bear on the decision. If you want to see pictures showing what Mounjaro pens and injections look like, that can help make the practical side of treatment feel more familiar before you start. If you are curious how tirzepatide and Mounjaro relate to each other as medicines, that page explains the connection between the drug name and the brand. For a broader view of what weight is a good starting point for Mounjaro, the eligibility guide goes through the BMI and comorbidity thresholds in detail. At nume, a real prescriber reads your consultation (not a queue for automated processing) and makes the recommendation based on your clinical picture. Which medicine suits you is a decision our prescribers make with you, not for you.