Wegovy After Mounjaro: How the Two Medicines Compare

The head-to-head trial that changed how clinicians think about these medicines

For years, people assumed Wegovy and Mounjaro were roughly equivalent — two injections, same idea, similar results. The SURMOUNT-5 trial, published in the New England Journal of Medicine in 2025, tested that assumption directly. In a randomised, open-label study of 751 adults with obesity but without type 2 diabetes, tirzepatide (at its highest tolerated dose) produced meaningfully greater average weight reduction over 72 weeks than semaglutide 2.4mg. That finding now sits at the centre of the clinical comparison between Wegovy and Mounjaro and informs how prescribers think about which medicine to offer first, and what happens when someone wants to switch.

NICE's appraisal of tirzepatide (TA1026, December 2024) also references indirect comparisons favouring tirzepatide, so the head-to-head data is consistent with the broader evidence reviewed by the committee. None of this makes Wegovy ineffective, semaglutide's STEP 1 trial, also in the New England Journal of Medicine, showed around 15% average weight reduction over 68 weeks, which is clinically significant. The gap simply matters when a patient hasn't reached their response on one medicine and is considering a change.

One thing the trial does not tell us: how individuals who have already been on Mounjaro will respond when moved to Wegovy, or vice versa. Cross-over data is limited. That gap in the evidence is exactly why prescriber involvement in any switch is not a formality.

Why someone might move from one to the other, and what the data suggests

There are a few legitimate reasons a clinician might consider switching direction. The most common is availability: supply pressures have affected both medicines at different points since 2023, and some patients have found themselves temporarily unable to continue their current treatment. Another reason is tolerability, a small number of people experience side effects on one GLP-1 that they do not on the other, though both share a broadly similar gastrointestinal profile (nausea, constipation, loose stools and reflux are the most frequently reported, usually most noticeable in the early weeks of a new dose). A third reason is cost: there is a notable price difference between Wegovy and Mounjaro in the UK private market, and for some patients that is a genuine factor.

What the evidence does not support is the idea that switching to Mounjaro after a plateau on Wegovy is automatically effective, or that going the other way preserves results. The SURMOUNT-5 data gives us a population-level picture; individual responses vary considerably. If you are thinking about moving to Mounjaro after a period on Wegovy, the prescriber will want to know your current weight, how long you have been on maintenance dose, and what your response looked like, the same evidence review that applies to a dose increase.

The misconception worth clearing up: some people assume the two medicines are interchangeable at equivalent doses, the way you might swap one statin for another. They are not. Tirzepatide activates two receptor pathways (GIP and GLP-1); semaglutide activates one. Different mechanisms, different titration schedules, different starting points. A switch is a clinical event, not a straight swap.

What UK licensing and NICE guidance say about eligibility when switching

Both medicines share broadly the same licensed eligibility criteria: adults with a BMI of 30 or above, or 27 to 29.9 with at least one weight-related condition such as high blood pressure, high cholesterol or obstructive sleep apnoea. Lower BMI thresholds can apply for some ethnic backgrounds under NICE guidance. Switching from one to the other does not reset your clinical eligibility, but it does require a fresh prescriber assessment, you are, in effect, starting a new treatment course with a different medicine.

NICE's recommendation for semaglutide (TA875) specifies that it should be used within a specialist weight management service for a maximum of two years. Tirzepatide (TA1026) has different NHS access criteria tied to an eligibility cohort rollout. Neither of those NHS-specific conditions applies to private prescribing, but they are worth knowing because they shape how NHS clinicians approach the switch question. The full side-by-side look at Wegovy versus Mounjaro covers the NHS and private access picture in more detail.

Patients considering beginning Wegovy after a course of Mounjaro, or the reverse, sometimes ask whether they need to wait between stopping one and starting the other. There is no universal washout period specified in UK guidance, but a prescriber will assess your current health status, any residual side effects, and your weight trajectory before issuing a new prescription. At nume, every repeat and every new prescription is reviewed by a named prescriber, not processed automatically, that applies to switches too.

How a switch decision is made in practice

A question our prescribers hear regularly is whether a patient who did not respond well to Wegovy will do better on Mounjaro, or whether someone who lost weight on Mounjaro can maintain on Wegovy if they need to change. The honest answer is that the evidence for those specific questions is thinner than the pivotal trial data. What we know is this: GLP-1 medicines as a class produce less effect once stopped, so staying on a medicine that works for you (rather than switching for reasons other than clinical need) is generally the better path.

If a switch is clinically appropriate, the prescriber will restart at the lowest dose of the new medicine. Titration begins again. That is not a step backward; it is how the medicines are designed to be introduced safely, and trying to jump to an equivalent dose carries a meaningfully higher risk of side effects. Guidance from NHS England on weight-management injections covers this general principle for anyone moving between treatments. The clinical picture of whether Wegovy is effective after Mounjaro, and the question of returning to Mounjaro after a period on Wegovy, are each covered in dedicated pages where you can explore the specifics.

Which medicine suits you (and in what direction any switch should go) is a clinical decision our prescribers work through with you. Start your free consultation and a GPhC-registered prescriber will review your case the same day.