Mounjaro®
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Start journey Learn moreWhat actually shapes the decision to switch — and how a prescriber works through it
Why there is no fixed washout window, and what fills that gap
A common assumption is that you must stop one medicine and wait several weeks before starting another. With many drug classes that logic holds, because overlapping mechanisms would cause harm. With Wegovy and Mounjaro the picture is more nuanced. Both act on GLP-1 receptors, which means their pharmacological footprints overlap considerably; adding one on top of the other at full dose is not appropriate. But a formal empty gap between them is not what current guidance demands either.
What matters instead is timing relative to your last dose and what dose level you are moving to. Semaglutide (Wegovy) has a half-life of roughly one week, meaning meaningful drug levels persist for several weeks after stopping. A prescriber switching you to tirzepatide (Mounjaro) will factor that in, typically starting the new medicine at its lowest initiation dose rather than an equivalent or higher one, and monitoring how your body adjusts. The specifics of moving from Wegovy to Mounjaro follow a slightly different logic to the reverse direction, because tirzepatide's dual GIP and GLP-1 action adds a variable the prescriber has to account for. NHS England's guidance on weight-management injections underlines that both transitions should happen under clinical supervision.
In short: the gap, if there is one, is decided clinically, not by a fixed calendar rule.
The factors your prescriber weighs before agreeing to a switch
Switching GLP-1 medicines is not a neutral act. A prescriber will want to understand why you are switching: a side-effect burden, a plateau in weight loss, availability, or a clinical preference based on your comorbidities. Each reason points to a different plan. Someone switching because of persistent nausea on Wegovy may benefit from a conservative start on Mounjaro, with the dose increase schedule slowed deliberately. Someone switching because they have reached the ceiling of Wegovy's effect may be moving to tirzepatide for its additional GIP pathway, which works differently and may produce a further response.
The dose you are on at the point of switching also matters. A person on the 2.4 mg Wegovy maintenance dose will have more semaglutide in their system than someone on 1.0 mg; the prescriber may time the first Mounjaro injection accordingly, rather than simply starting on day one after the last Wegovy pen. If you are considering this change, the clinical reasoning behind choosing between the two medicines is worth reading before your consultation, so you arrive with the right questions ready.
For a direct comparison of how these two medicines performed in trials, including the SURMOUNT-5 head-to-head study published in the New England Journal of Medicine (2025), see the Wegovy vs Mounjaro evidence page.
How the two medicines compare on the points most relevant to switching
The table below covers the clinical characteristics that matter most when a prescriber is planning a transition. Numbers are drawn from published trial data and NICE appraisals.
| Factor | Wegovy (semaglutide) | Mounjaro (tirzepatide) |
|---|---|---|
| Mechanism | GLP-1 receptor agonist | Dual GIP and GLP-1 receptor agonist |
| Half-life (approx.) | ~1 week | ~5 days |
| Average weight loss in trials | ~15% at 2.4 mg (STEP 1, 68 weeks) [NEJM] | ~20–21% at 15 mg (SURMOUNT-1, 72 weeks) [NEJM] |
| NICE-recommended for weight loss | Yes (TA875 (specialist services, max 2 years) | Yes) TA1026 (published December 2024) |
| Side-effect profile | GI-led: nausea, vomiting, diarrhoea, constipation | GI-led: broadly similar profile; individual tolerance varies |
| Starting dose when switching | Clinical decision; typically re-initiation dose | Clinical decision; typically 2.5 mg initiation dose |
Individual responses vary considerably. Neither medicine suits everyone, and the trial averages above do not predict what any one person will experience. There is also a cost dimension to the choice: the cost comparison between Wegovy and Mounjaro is worth reviewing if affordability affects the decision. Which medicine suits you is a clinical decision our prescribers make with you.
What to do if you are ready to explore switching
If you are already on one of these medicines and thinking about moving to the other, the most straightforward step is a prescriber review. At nume, every repeat and every change is reviewed by a GPhC-registered Independent Prescriber before anything is dispensed, your previous treatment history is part of that assessment. You can read about how our clinical team approaches decisions like this on the clinical team page, and find general treatment options at weight-loss treatments.
If you are on a different medicine elsewhere and want to transfer to Mounjaro or Wegovy through nume, you will be asked to provide evidence of your current treatment, including the dose you are on. That step protects you: a prescriber cannot safely plan a switch without knowing your starting point. Questions about the process are covered in the FAQs, or you can reach us directly through the contact page. For a broader look at what switching between these two medicines actually involves day to day, the guide on whether a break is needed goes into practical detail that complements this page.
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