Mounjaro®
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Start journey Learn moreMechanism, evidence and what the switch actually involves at 15 mg
The myth: Mounjaro 15 mg and Wegovy 2.4 mg are roughly equivalent because 15 is bigger than 2.4
Numbers on medicine labels carry no cross-drug meaning. The doses of tirzepatide and semaglutide were chosen independently in separate clinical programmes, calibrated to each medicine's own pharmacology. A patient finishing the Mounjaro titration ladder at 15 mg has not necessarily reached the ceiling of Wegovy's effect, and vice versa.
The evidence underlines the gap. In the SURMOUNT-5 head-to-head trial (the first direct comparison, published in the New England Journal of Medicine in 2025) tirzepatide produced greater average weight reduction than semaglutide 2.4 mg over 72 weeks in adults with obesity and no diabetes. Meanwhile, Novo Nordisk's higher 7.2 mg semaglutide dose narrows the gap between the two medicines. That single data point shows how fluid the comparison becomes the moment dosing changes on either side.
If you are currently at Mounjaro 15 mg and your prescriber is considering Wegovy, the relevant clinical question is which semaglutide dose would be appropriate to start, not which number matches. Our dose conversion guide explains how prescribers approach that judgement, but the decision is always individual.
What the evidence actually shows for each medicine at its highest licensed dose
Comparing the two at their respective maximums gives the clearest picture of what each can achieve. The table below draws from the pivotal trial results cited in NICE guidance.
| Factor | Mounjaro 15 mg (tirzepatide) | Wegovy 2.4 mg (semaglutide) |
|---|---|---|
| Average weight reduction (trial) | ~20–21% over 72 weeks (SURMOUNT-1) | ~15% over 68 weeks (STEP 1) |
| Receptor targets | GIP and GLP-1 | GLP-1 only |
| UK licence for weight management | Yes (BMI ≥30, or ≥27 with a weight-related condition) | Yes (BMI ≥30, or ≥27 with a weight-related condition) |
| Injection frequency | Once weekly | Once weekly |
| NICE recommended for NHS use | Yes (TA1026, Dec 2024 (BMI ≥35 + comorbidity, phased rollout) | Yes) TA875, Mar 2023 (specialist services, max 2 years) |
Sources: NICE TA1026, recommendations chapter; NICE TA875; SURMOUNT-1 and STEP 1 trial data. Trial averages describe the study populations, individual results vary.
The 15 mg tirzepatide figure comes from a trial of around 2,500 adults (SURMOUNT-1 alone). The semaglutide figure reflects STEP 1, a similarly powered programme. Both medicines are Black Triangle (▼) products under additional MHRA monitoring, which is routine for newer medicines rather than a signal of unusual concern. For a side-by-side look at the full range of Wegovy and Mounjaro strengths, the Wegovy vs Mounjaro dose overview covers each step of both titration schedules.
When a switch from Mounjaro 15 mg to Wegovy might come up, and what it means practically
A switch at the top of the tirzepatide ladder is less common than a switch at lower doses, simply because 15 mg is the endpoint most patients are working toward, not away from. The scenarios where it arises are usually supply-related (a temporary stock issue), a prescriber's clinical preference based on your response profile, or a patient's own preference after weighing the options.
Switching medicines at this point is not the same as continuing at an equivalent intensity. A prescriber would typically restart the semaglutide titration cautiously, often from a lower dose, to manage GI tolerability, this is not a step backward; it reflects how differently the two molecules behave. If you are considering the journey in that direction, our guide to moving from 1 mg Wegovy to Mounjaro also helps illustrate how titration decisions are made at each stage of a switch. The 2.4 mg Wegovy to Mounjaro page covers the reverse journey if context is helpful.
On the practical side: Wegovy arrives as a pre-filled injection pen, once weekly, stored in the fridge. Patients who switch from Mounjaro will notice the pens look and feel different but the injection routine (rotating sites between abdomen, thigh and upper arm) is the same. If you are currently on Mounjaro 15 mg and want to discuss your options, our clinical team reviews every consultation the same day. Worth noting: a switch of this kind always requires fresh clinical assessment, not just a new order.
Which is right for you, and how to move forward
There is no universal answer to whether staying on Mounjaro 15 mg or switching to Wegovy produces better outcomes for any individual. The SURMOUNT-5 data favours tirzepatide on average weight loss, but averages describe populations, not people. Tolerability, medical history, personal preference about the injection experience and practical factors all influence what works best.
Both medicines are prescription-only. That matters more than any headline figure: the right choice is reached through a clinical conversation, not a comparison article. Our prescribers review the evidence alongside your history at consultation, and the starting dose for any switch is set to suit you rather than lifted from a table. If cost is part of the decision, a cost comparison between the two medicines sets out the private-market picture honestly.
Patients currently on Mounjaro who want to understand the full range of their options can also read the broader Wegovy vs Mounjaro comparison or visit the weight-loss treatment overview before starting a consultation. Which suits you is a clinical decision our prescribers make with you, not something a page like this can settle.
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