Switching from Wegovy to Mounjaro: What the Washout Period Actually Means

Step 1: Understanding why a washout period is often assumed, and why it usually isn't needed

The assumption comes from a reasonable instinct: when you stop one medicine and start another, surely the first needs clearing from your system. For some drug classes that logic holds. With GLP-1 medicines it is more nuanced. Semaglutide (Wegovy) has a long half-life of roughly one week, which is precisely why a single weekly injection sustains a steady level in the blood. Tirzepatide (Mounjaro) has a similar half-life. Because both medicines work on overlapping appetite pathways, there is no pharmacological clash that demands a gap, and a prolonged gap would simply mean weeks without any treatment, which most people want to avoid.

What a prescriber will think about instead is whether your side-effect profile, your blood-glucose history, and your reason for switching all point in the same direction. The NHS England guidance on weight management injections does not specify a mandatory washout gap for switching between these medicines. If you want to understand what the washout period between Wegovy and Mounjaro actually involves and when it may or may not apply, our dedicated page walks through the clinical reasoning in full. Worth knowing before you assume you need to stop for weeks first.

Step 2: What your prescriber will want to know before approving the switch

A switch is not a straightforward reorder. At nume, a named prescriber reviews every case individually, your current dose, how long you have been on it, your reason for switching, and any side effects you have had. If you were on a higher semaglutide dose, your prescriber will assess which tirzepatide starting dose suits you; going straight to a high tirzepatide dose without that assessment carries a real gastrointestinal risk. The NHS tirzepatide licence covers adults with a BMI of 30 or above, or 27 or above with at least one weight-related condition such as high blood pressure, high cholesterol, or prediabetes. Lower BMI thresholds can apply for some ethnic backgrounds under UK guidance.

Evidence of your current treatment matters too. Prescribers at a regulated pharmacy need to see that you have genuinely been established on Wegovy before approving the next step, this protects you as much as it protects the clinical record. Our clinical team follows this process on every transfer case, no exceptions.

Step 3: The comparison, how Wegovy and Mounjaro differ on the key dimensions

Semaglutide is a GLP-1 receptor agonist. Tirzepatide activates both GLP-1 and GIP receptors, making it the only dual-agonist weight-loss medicine currently licensed in the UK. In the SURMOUNT-5 trial, published in the New England Journal of Medicine in 2025, tirzepatide produced greater average weight loss than semaglutide 2.4mg over 72 weeks in adults with obesity and no diabetes. The SURMOUNT-1 trial recorded around 20–21% average body-weight reduction at the 15mg dose. STEP 1, the pivotal semaglutide trial, found approximately 15% average reduction at 2.4mg over 68 weeks, as reported in the NEJM. Wegovy's newer 7.2mg dose, approved by the MHRA in January 2026, narrows that gap. Neither medicine is a guaranteed outcome; these are group averages from controlled trials.

For a fuller side-by-side breakdown of outcomes, cost context, and eligibility, our Wegovy vs Mounjaro comparison page covers the detail across all the key dimensions. If you are thinking about it from the other direction, our guide to switching from Mounjaro to Wegovy applies the same framework in reverse, and if you want a direct head-to-head on which medicine may suit you better, our Wegovy vs Mounjaro page works through the key differences to help you decide.

Step 4: Starting the switch through a regulated pharmacy

Once your prescriber has reviewed your history and confirmed you are a suitable candidate, the process at nume is straightforward. You complete a free consultation online, upload evidence of your current treatment, and a GPhC-registered prescriber reads it that day. If the switch is clinically appropriate, your prescription is written and your Mounjaro pen is dispatched the same day (provided you order before 12pm Monday to Friday) arriving free by DPD the next working day. Our pharmacy is GPhC-registered (registration number 9012878), and medicines move through the licensed UK supply chain.

The one thing worth saying plainly: which medicine is right for you is a clinical decision, not a consumer one. Our prescribers make it with you, not for you, not instead of you. Speak to our prescribers if you are ready to explore the switch, or browse our treatment overview if you want more context first. If you have questions about the process, the FAQs page may already answer them.