Mounjaro or Wegovy: which is more effective for weight loss?

The biggest misconception: that one of these medicines is simply 'the stronger one'

Most people arrive at this comparison having already concluded Mounjaro wins and they just need confirmation. The reality is messier. Yes, in SURMOUNT-5, a 72-week open-label trial of 751 adults with obesity and no diabetes, tirzepatide produced greater average body-weight reduction than semaglutide 2.4mg — the SURMOUNT-1 data published in the New England Journal of Medicine had already shown tirzepatide achieving around 20–21% average loss at 15mg, while the STEP 1 trial reported around 15% for semaglutide 2.4mg over 68 weeks. Those numbers are real. But 'stronger on average in a clinical trial' and 'more effective for you personally' are different questions.

Both medicines act on GLP-1 receptors to slow gastric emptying and reduce appetite. Tirzepatide adds a second mechanism, activating GIP receptors as well — the only dual-agonist weight-loss medicine licensed in the UK. Whether that second pathway makes a meaningful difference for any individual depends on biology that a prescriber has to assess. Some people respond more than the trial average to semaglutide; some people tolerate one medicine better than the other, which matters enormously for real-world results. If you want a closer look at what is more effective, Mounjaro or Wegovy, the evidence is more nuanced than headline trial figures suggest. The numbers from trials describe populations, not individuals.

If you want to go deeper on how the results compare across specific studies, our detailed breakdown of Wegovy's effectiveness versus Mounjaro covers the trial designs side by side.

A fair comparison: what the data table actually shows

Here is what verified UK sources and published trial data show for the two licensed weight-management injections.

One detail worth noting: the MHRA approved a single-dose 7.2mg Wegovy pen in April 2026, with trials reporting around 20.7% average weight loss at that dose over 72 weeks. That narrows the gap with tirzepatide 15mg considerably. It is still a higher dose than the 2.4mg that most people will have seen quoted, and its long-term head-to-head data against tirzepatide at equivalent doses remains limited.

The cost comparison between the two is a separate question worth reading before you decide, price has shifted significantly since Eli Lilly revised Mounjaro's UK list price in September 2025.

Side effects, routine and what actually affects your result

Both medicines share a broadly similar side-effect profile: nausea, loose stools, constipation, reflux and fatigue are the most common, usually most pronounced in the first few weeks of a new dose. For most people these settle; for some they don't, and switching medicines is sometimes the clinical answer. That is a decision made with a prescriber, not by reading a comparison article.

Practically speaking, both are once-weekly subcutaneous injections. Both pens are kept in the fridge (slotted alongside whatever else lives in the fridge door) and taken out briefly before each weekly injection. Wegovy tablets (oral semaglutide, approved by the MHRA in June 2026) offer a needle-free option for those who prefer it, though the trial data differs from the injection. If injection anxiety is part of the decision, that is worth raising in your consultation.

Women using oral contraceptives starting tirzepatide should add a non-oral contraceptive method for the first four weeks of treatment and for four weeks after each dose increase, because absorption of the pill may be affected. NHS England's guidance on weight-management injections covers this and other practical considerations including HRT and surgery.

Our guide on switching from semaglutide to tirzepatide is useful if you are already on Wegovy and wondering whether tirzepatide might work better for you. For a broader view of both medicines in context, the Wegovy versus Mounjaro overview is a good starting point.

Which one should you actually take?

Neither this page nor any comparison article can answer that. A prescriber reviewing your BMI, weight-related conditions, medication history, contraceptive use and preferences can. That is precisely the assessment our clinical team carries out before any prescription is issued, no algorithm, just a named prescriber reading your answers the same day you submit them.

Both medicines are covered by NICE's appraisal of tirzepatide (TA1026) and TA875 for semaglutide, meaning the evidence base for both has been formally evaluated at the highest UK level. The NICE thresholds, your medical history and how your body responds all feed into the clinical picture. Lower BMI thresholds can apply for some ethnic backgrounds under UK guidance, another reason a personalised assessment matters.

If you are trying to decide between the two before your consultation, the which is better page covers the practical factors our prescribers typically weigh up. Or explore our weight-loss treatment overview to see both options together. When you are ready, speak to our prescribers, the consultation is free, and the clinical team will recommend whichever medicine is the right fit for you, or neither if neither is appropriate.