Mounjaro®
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Start journey Learn moreHow semaglutide and tirzepatide compare: mechanisms, trial data, and what switching actually involves
The myth: you can convert your semaglutide dose to a tirzepatide equivalent by weight
It is a reasonable-sounding idea. You are on 1.7mg semaglutide weekly, you want to switch to tirzepatide, so you look for the matching dose. The problem is that the two drugs do not share a common unit of biological effect. Semaglutide activates one receptor pathway (GLP-1) at doses measured in single-digit milligrams. Tirzepatide activates two pathways simultaneously (GLP-1 and GIP) at doses that look larger on paper (2.5mg to 15mg) but cannot be compared directly to semaglutide's scale because the molecular targets, receptor affinities and dose-response curves are fundamentally different. A milligram of tirzepatide is not a milligram of semaglutide plus a top-up; it is a different molecule doing a different job at a receptor level.
Any semaglutide to tirzepatide dose conversion chart you find circulating online has been constructed by inference, not by clinical trial or regulatory authority. Neither NICE, the MHRA, nor NHS England publishes conversion guidance, because the concept does not map cleanly onto the pharmacology. Our overview of semaglutide-equivalent dosing for tirzepatide explains the pharmacological reasoning in more detail if you want to understand the science behind this.
What the head-to-head trial data actually tells us
The SURMOUNT-5 trial, published in the New England Journal of Medicine in 2025, compared tirzepatide directly against semaglutide 2.4mg weekly in 751 adults with obesity but without type 2 diabetes, over 72 weeks. Tirzepatide produced a greater average reduction in body weight across that period. For context, SURMOUNT-1 reported average weight loss of around 20–21% at the 15mg tirzepatide dose over a similar timeframe, while STEP 1 (the equivalent landmark trial for semaglutide 2.4mg) reported around 15% over 68 weeks. These are population averages in controlled trial conditions; individual results vary considerably.
The approved higher semaglutide dose of 7.2mg (the dedicated single-dose pen approved by the MHRA in April 2026) narrows that gap, with trials reporting around 20.7% average weight loss at 72 weeks — closer to tirzepatide's headline figures. Neither medicine is the automatic right choice; the question is which suits a particular person's circumstances, history and tolerance. That is a clinical judgement, not a data-matching exercise. Our prescribers discuss both options in detail during your consultation, see the treatment options page for an overview of what is available.
| Feature | Semaglutide (Wegovy injection) | Tirzepatide (Mounjaro) |
|---|---|---|
| Receptor targets | GLP-1 only | GLP-1 and GIP (dual agonist) |
| UK licensed doses | 0.25mg → 2.4mg weekly (7.2mg approved Apr 2026) | 2.5mg → 15mg weekly |
| Average trial weight loss (maintenance dose) | ~15% at 2.4mg (STEP 1, NEJM); ~20.7% at 7.2mg | ~20–21% at 15mg (SURMOUNT-1, NEJM) |
| Head-to-head evidence | SURMOUNT-5 (NEJM, 2025): tirzepatide produced greater average weight reduction vs semaglutide 2.4mg over 72 weeks | |
| UK licence (weight management) | BMI ≥30, or ≥27 with a weight-related condition | BMI ≥30, or ≥27 with a weight-related condition |
| Starting dose when switching | Prescriber-determined; not derived from prior tirzepatide dose | Prescriber-determined; not derived from prior semaglutide dose |
Sources: SURMOUNT-1, New England Journal of Medicine; STEP 1, New England Journal of Medicine; NICE technology appraisal TA1026 (tirzepatide).
What a safe switch from semaglutide to tirzepatide actually looks like
In practice, switching means stopping semaglutide and beginning tirzepatide at an appropriate starting dose, decided by a prescriber based on your current weight, how long you have been on semaglutide, how well you tolerated it, and what your goals are. There is no agreed washout period mandated by the Mounjaro SmPC for switching from semaglutide specifically, but a prescriber will consider the timing of your last semaglutide dose and the potential for overlapping GI effects. Some people switch with minimal fuss; others find the adjustment requires careful dose management. Starting at the lowest tirzepatide dose is common, though not universal, your prescriber decides.
The practical routine shifts too. Semaglutide lives in the fridge at home (and many people keep it on the fridge door next to everyday items); tirzepatide also requires refrigeration and each pen lasts four weeks rather than one. It is a different injection rhythm, which some people prefer. For a closer look at the specific semaglutide doses involved in a switch, our pages covering switching from 2.4mg semaglutide and switching from 1.7mg semaglutide go deeper on those starting points. There is also broader context on the practicalities of switching dosage from semaglutide to tirzepatide if you want the full picture before your consultation.
One thing that does not change: this is a prescription medicine, and any switch must be assessed by a prescriber. A question our prescribers field most weeks is whether someone who has plateaued on semaglutide might do better on tirzepatide, and the answer genuinely depends on the individual. Which suits you is a clinical decision our prescribers make with you, not one a conversion chart can make for you. If you are considering a switch, exploring your treatment options is a good first step, and the reverse question of Mounjaro to Wegovy conversion follows the same clinical logic if you are switching in the other direction.
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