Tirzepatide or Mounjaro: is there actually a difference, and which is better for weight loss?

The trial data that settled most of the debate

The SURMOUNT-1 trial, published in the New England Journal of Medicine, randomised 2,539 adults with obesity to tirzepatide or placebo over 72 weeks. Average body-weight reductions at the 15mg dose were around 20–21%, with some analyses reaching approximately 22.5% among participants without type 2 diabetes. Those are figures that had not been seen before for a licensed medicine in this class. The trial was large, rigorous and published in a peer-reviewed journal, which is why NICE leaned heavily on it when issuing its recommendation for tirzepatide in December 2024.

The mechanism helps explain the results. Most GLP-1 medicines act on a single receptor pathway. Tirzepatide adds a second: it also activates GIP receptors, which play a separate role in appetite signalling and fat metabolism. Activating both pathways together appears to produce a larger appetite-reduction effect than either alone. The clinical data support that picture, though the exact contribution of each receptor is still being studied.

A question our prescribers hear most weeks is whether the higher efficacy numbers from the trials translate to real-world results. The honest answer is that averages mask wide individual variation. Some people lose considerably more than 20%; others find the lower doses their body tolerates are already producing meaningful change. The trial figures are the best available guide, not a personal guarantee.

How tirzepatide compares to semaglutide, and why the comparison matters here

When someone types "tirzepatide or Mounjaro which is better," they sometimes mean: better than semaglutide (Wegovy)? That comparison now has direct evidence. The SURMOUNT-5 trial, published in the New England Journal of Medicine in 2025, ran tirzepatide head-to-head against semaglutide 2.4mg in 751 adults with obesity over 72 weeks. Tirzepatide produced greater average weight loss. NICE's committee discussion in TA1026 also noted that indirect comparisons favoured tirzepatide over semaglutide at standard maintenance doses.

It is worth being precise about what that means in practice. Novo Nordisk has since received MHRA approval for a 7.2mg semaglutide dose, which trials reported produced around 20.7% average weight loss, narrowing the gap with tirzepatide's top-dose results considerably. Neither medicine is definitively "better" for every individual; tolerability, how quickly side effects settle, injection frequency and personal response all feed into which suits any given person, and our page on how tirzepatide and Mounjaro relate to each other unpacks that distinction in more detail. That is a clinical conversation, not a ranking exercise.

For context on how other emerging medicines sit relative to tirzepatide, the page on whether anything currently outperforms tirzepatide covers the pipeline in more detail, including whether cagrisema is better than tirzepatide for those following the latest trial data. There is also a direct breakdown for anyone looking at how liraglutide compares to tirzepatide.

Who tirzepatide is licensed for, and what the prescriber is weighing up

In the UK, Mounjaro is licensed for adults with a BMI of 30 or above, or a BMI of 27 or above alongside at least one weight-related condition such as high blood pressure, type 2 diabetes, high cholesterol or obstructive sleep apnoea. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance. The prescriber considers the full clinical picture: current medicines, relevant medical history, and whether any contraindications apply.

Tirzepatide carries a Black Triangle designation, meaning the MHRA is collecting additional safety data as post-market evidence builds. Common side effects are largely gastrointestinal (nausea, loose stools, constipation, indigestion) typically most noticeable after starting or after a dose increase, and usually settling within one to two weeks. The NHS medicines page for tirzepatide lists the full side-effect profile and the symptoms that should prompt urgent medical attention.

Tirzepatide is not licensed for use in pregnancy, while breastfeeding, or for anyone under 18. A history of certain thyroid conditions or pancreatitis requires careful prescriber assessment. Because it can slow gastric emptying, women taking oral contraceptives are advised to use an additional non-oral method for the first four weeks of treatment and for four weeks after any dose increase, as pill absorption may be reduced. Anyone also curious about how newer pipeline medicines such as retatrutide fit into this picture can find a detailed comparison on our page covering whether retatrutide or Mounjaro is the better option.

If you are weighing up the treatments available at nume and want to see what a consultation involves, the Mounjaro overview page covers the full picture, and for a broader look at all licensed weight-loss options, the treatment overview is a useful starting point before you speak to a prescriber. Questions about cost are addressed transparently on the treatment and pricing page.