Mounjaro worked — so why isn't Wegovy working the same way?

The myth worth correcting first: all GLP-1 medicines are interchangeable

A common assumption is that Mounjaro and Wegovy are essentially the same drug in different packaging. Both slow gastric emptying, both reduce appetite, and both sit in the broad GLP-1 family, so if one works, the other should too. That logic is understandable, but it misses something important about how the two medicines actually act.

Wegovy (semaglutide) activates the GLP-1 receptor alone. Mounjaro (tirzepatide) activates both the GLP-1 receptor and the GIP receptor, a second gut-hormone pathway involved in appetite regulation and fat metabolism. That dual action is why tirzepatide is described as a dual agonist and why, in the SURMOUNT-5 head-to-head trial published in the New England Journal of Medicine in 2025, tirzepatide produced meaningfully greater average weight loss than semaglutide 2.4 mg over 72 weeks in adults with obesity and no diabetes. The added GIP signal appears to do work that GLP-1 alone does not replicate. So switching from the more potent mechanism to the less potent one and finding the results softer is not a personal failure, it reflects the underlying pharmacology. If you are reading more about this comparison, the Wegovy vs Mounjaro overview on this site covers the mechanism and evidence in fuller detail.

There is also the question of dose equivalence. Wegovy's standard maintenance dose of 2.4 mg may not represent the same degree of receptor engagement your body had become accustomed to on Mounjaro 10 mg or 15 mg. The licensed dose ranges are not directly comparable on a milligram basis.

What the head-to-head evidence actually shows

The SURMOUNT-5 trial is the most clinically relevant reference point here. It randomised 751 adults with obesity and no type 2 diabetes to tirzepatide or semaglutide 2.4 mg for 72 weeks, making it the only published head-to-head randomised controlled trial of the two medicines at their weight-management doses. Tirzepatide produced greater average percentage body-weight reduction. NICE's appraisal of tirzepatide (TA1026) also noted, in its committee discussion of indirect comparisons, that the weight-loss evidence favours tirzepatide, a finding that contributed to its recommendation for NHS use.

The table below summarises the key differences factually.

These numbers come from different trial populations and different protocols, so direct comparison carries caveats, but they reflect the best available evidence. For a closer look at how costs sit alongside these differences, the cost comparison page breaks down what people typically pay for each.

Reasons Wegovy might be underperforming for you specifically

Even accepting the trial-level difference, individual responses vary. A few factors are worth discussing with your prescriber.

Dose timing matters. If you moved from a high Mounjaro dose to the early rungs of the Wegovy schedule (0.5 mg or 1.0 mg) it is too soon to judge. Semaglutide's maintenance dose is 2.4 mg, and many of its effects are dose-dependent. Titration takes months.

Tolerance and appetite rebound also play a role. Some people find that after an extended period on a GLP-1 medicine their appetite suppression feels less pronounced when they switch medicines, at least temporarily. This may settle. It may not. That is genuinely an individual question a prescriber needs to assess with you, and if you have moved across from tirzepatide specifically, our detailed guide to Wegovy not working after Mounjaro looks at exactly why this happens and what your options are. The question of what happens to weight if you stop either medicine altogether is separate but worth knowing about, the evidence on weight regain after stopping treatment is sobering for both medicines.

Absorption differences are worth a brief mention. Wegovy is a subcutaneous injection, as is Mounjaro. But injection technique, site rotation and storage all affect how consistently the medicine reaches your system. If your pen has been stored outside the fridge for longer than the leaflet permits, that is a concrete, fixable variable.

When to speak to a prescriber, and what that conversation looks like at nume

If you had a clear response on Mounjaro and Wegovy is not matching it, the right move is a clinical conversation rather than waiting longer in hope. A prescriber can review whether switching back to tirzepatide is appropriate, whether dose adjustment on semaglutide makes sense, or whether something else is affecting your response.

At nume, that conversation starts with a free consultation. A named prescriber (not a chatbot, not an algorithm) reviews your answers and your treatment history the same day. If a change in treatment is clinically suitable, your new pen is dispatched the same day you order before 12pm, arriving in plain packaging via DPD with a tracking link so you know exactly when to expect it. For people already on treatment elsewhere who want to transfer, our guide for patients who switched from Mounjaro to Wegovy and found it not working outlines what evidence you would need to bring.

If you are weighing whether to try semaglutide for the first time or return to tirzepatide, the page covering that specific decision looks at it from the other direction. And if you want a broader view of all the weight-loss treatment options we offer, the treatment overview is a good place to start before booking.

Which medicine suits you is a clinical decision our prescribers make with you, based on your history, your current weight trajectory and any other health factors. The trial data gives us the population picture; your prescriber fills in the individual one. Start your free consultation and have that conversation today.