Should I use Wegovy or Mounjaro? Here's what the evidence actually says

The biggest myth: that these two medicines are basically the same thing

A lot of people arrive at this question assuming Wegovy and Mounjaro are interchangeable, two brand names for what is essentially one drug, priced differently. That is not quite right, and the difference matters clinically.

Wegovy contains semaglutide, a GLP-1 receptor agonist that works on one appetite-regulating pathway. Mounjaro contains tirzepatide, which activates two receptors at once (both GLP-1 and GIP) making it the only dual-agonist weight-loss medicine currently licensed in the UK. That extra mechanism appears to translate into meaningfully greater average weight loss in the trial data. In SURMOUNT-1, participants on tirzepatide 15mg lost an average of around 20–21% of body weight over 72 weeks, as reported in the New England Journal of Medicine. The STEP 1 trial found semaglutide 2.4mg produced around 15% average weight loss over 68 weeks. Those are population averages from controlled conditions, individual results vary.

Then came SURMOUNT-5: a direct head-to-head trial published in the NEJM in 2025 comparing tirzepatide to semaglutide 2.4mg in 751 adults with obesity and no diabetes. Tirzepatide produced greater average weight reduction. Importantly, the newer 7.2mg semaglutide dose (MHRA-approved for a dedicated single-dose pen in April 2026) appears to narrow that gap, though no head-to-head data exists for it yet. If you are weighing up whether Wegovy or Mounjaro is the right choice for you, you can also explore a fuller side-by-side breakdown on our Wegovy vs Mounjaro comparison page.

The point is not that one medicine is universally superior. It is that the two medicines are genuinely different, and your prescriber weighs the evidence alongside your specific situation.

How the two medicines compare on the things patients actually ask about

One practical note on cost: private treatment prices vary by dose and provider. If price is part of your thinking, our cost comparison page for Wegovy and Mounjaro walks through what drives the difference. Neither medicine should be chosen primarily on price, clinical fit comes first.

Side effects, eligibility and the things worth discussing before you decide

The side-effect profiles of both medicines are broadly similar: nausea, constipation, diarrhoea, indigestion and fatigue are most common, particularly in the weeks after starting or after a dose step-up. These tend to settle. Injection-site reactions can occur with either. Neither should be used in pregnancy, while breastfeeding, or if you are actively trying to conceive, and both are licensed for adults only. History of certain conditions (medullary thyroid carcinoma, MEN2, pancreatitis) is a reason to discuss carefully with a prescriber before starting either one.

For women taking the oral contraceptive pill, tirzepatide specifically may reduce pill absorption in the first four weeks of treatment and for four weeks after each dose increase; adding a non-oral method during those windows is the current NHS recommendation. Semaglutide does not carry the same evidence of pill interaction, though individual circumstances vary. If you are thinking about switching from one to the other, our page on switching from semaglutide to tirzepatide covers what that process looks like in practice.

On eligibility: the private licensed thresholds are the same for both medicines, BMI of 30 or above, or 27 or above with at least one weight-related condition such as high blood pressure, type 2 diabetes, high cholesterol or obstructive sleep apnoea. Lower BMI thresholds can apply for some ethnic backgrounds under UK guidance. A qualifying BMI on its own does not confirm suitability; a prescriber assesses the full picture.

The NICE appraisal of tirzepatide (TA1026) and the NICE appraisal of semaglutide (TA875) both set out NHS criteria, which are narrower than private eligibility, useful context if you are also considering the NHS route. For a broader look at what treatment might suit your situation, our weight-loss treatment overview is a reasonable starting point.

So which one is right for you?

A question our prescribers hear most weeks: "just tell me which one is better." The honest answer is that the trials favour tirzepatide on average weight loss, but averages don't tell you what will happen for you specifically. Semaglutide has a longer track record. Some people respond better to one than the other. Some people have health reasons that make one less suitable. Payday timing, a Monday order deadline, bank holidays, none of that should drive the choice.

If you have already been on one and are wondering whether switching makes sense, the page on switching from Mounjaro to Wegovy and its counterpart on choosing between Mounjaro and Wegovy go into more detail on the scenarios. Our clinical team, led by our prescribers, can also answer specific questions, see our clinical team page for more on who reviews your consultation.

Which medicine suits you is a clinical decision our prescribers make with you, based on your health, your history, and the most current evidence. The starting point is a free consultation.