Why Wegovy may not seem to be working after you switched from Mounjaro

The pharmacology gap that explains the difference in response

Mounjaro (tirzepatide) activates two gut-hormone receptors simultaneously: GIP and GLP-1. Wegovy (semaglutide 2.4mg) targets GLP-1 alone. That single-versus-dual distinction is not a marketing nuance — it appears to produce a measurably different physiological response in many people. In SURMOUNT-1, published in the New England Journal of Medicine, adults taking tirzepatide 15mg lost around 20–21% of their body weight on average over 72 weeks. STEP 1, the equivalent semaglutide 2.4mg trial, showed roughly 15% average loss over 68 weeks. These are population averages across thousands of participants; individual responses vary considerably.

The SURMOUNT-5 head-to-head trial, published in the New England Journal of Medicine in 2025, put both medicines directly against each other in adults with obesity and no diabetes. Tirzepatide produced greater average weight reduction. A fuller breakdown of that head-to-head is here. The practical implication for someone who had been responding well to Mounjaro is that Wegovy's single-pathway action may simply not replicate the appetite suppression they were used to — not because Wegovy is ineffective, but because tirzepatide's additional GIP activity gave their physiology something extra.

None of this means the switch cannot work. It means expecting identical results from the outset is not well-supported by the evidence, and any comparison of your progress should factor in the different mechanisms at play. The NICE appraisal of tirzepatide sets out the clinical evidence base in full.

Why the timing of the switch matters as much as the medicine itself

Semaglutide starts at 0.25mg and climbs through 0.5mg, 1.0mg, 1.7mg and 2.4mg over several months. That titration exists to manage tolerability, not to deliver therapeutic effect at the early steps. Someone who was stable on Mounjaro 10mg or 15mg and then restarts at 0.25mg Wegovy is, in clinical terms, back at a starter dose, and a starter dose of a single-agonist medicine at that.

This is the most common reason people feel Wegovy is not working: they are comparing the result from their final Mounjaro dose to the result from Wegovy's opening dose. The timelines simply do not match. It can take four to six months to reach and settle at Wegovy's 2.4mg maintenance dose, and that is before the body has time to respond to it steadily. If you are currently early in the titration, the question of whether Wegovy is working for you cannot yet be answered by the scales.

There is also the question of how long you waited between stopping Mounjaro and starting Wegovy. Read about recommended washout and crossover timing here. Starting too soon or leaving too long a gap can each disrupt the transition in different ways, and prescriber guidance on this matters.

Practically speaking, if the switch happened around a period of disruption (holidays, a change in eating pattern, or even just a busy week where the pen sat in a bag rather than the fridge) those factors layer onto what is already a pharmacologically different medicine at a lower dose. Worth keeping in mind.

How Mounjaro and Wegovy compare on the measures that matter most to switchers

The table below summarises the key clinical differences. These are the figures most relevant if you are trying to understand why your experience on the two medicines differs.

Numbers from published trials; individual results differ. Source: STEP 1, New England Journal of Medicine; SURMOUNT-1 and SURMOUNT-5, NEJM.

What to do if you genuinely believe Wegovy is underperforming for you

First, establish where you actually are in the titration before drawing conclusions. If you are still stepping up through doses, the prescriber's question is whether to continue or to consider other factors, not whether to declare the medicine a failure. If you are at 2.4mg maintenance and several months in with no meaningful progress, that is a different conversation, and one worth having with your prescriber rather than attempting to solve alone.

Switching back to Mounjaro is clinically possible, and some people do return to tirzepatide after finding semaglutide insufficient. Whether that is appropriate for you depends on why the original switch happened (supply availability, cost, side-effect profile) and whether those reasons still apply. Our clinical team reviews every case individually.

It is also worth understanding that stopping either medicine without a plan carries its own risk. Research shows that most people regain a substantial proportion of lost weight after stopping GLP-1 treatment without lifestyle and medical support in place. Stalling is not the same as failing, and if you are finding that Wegovy is not working after Mounjaro, there are specific reasons that pattern is common and steps worth working through before considering a stop. Cost is sometimes behind these decisions; a cost comparison of the two medicines may help if that is part of what you are weighing up.

Which medicine suits you, at what dose and for how long, is a clinical decision. At nume, a GPhC-registered prescriber reads your consultation personally and works through these questions with you. Check your eligibility and start your free consultation here.