No official conversion exists. Not from the FDA, not from the EMA, not from the MHRA or NICE, and not from either manufacturer. The conversion charts circulating online are someone’s clinical opinion presented as though it were labelling.
Why there cannot be one
Semaglutide is a GLP-1 receptor agonist. Tirzepatide acts on two receptors — GIP as well as GLP-1. There is no shared pharmacological unit between them, so milligram-for-milligram equivalence is not a definable quantity. It is not that nobody has got round to working it out; there is no quantity there to work out.
You can see this in the trial data. In SURMOUNT-5, a 72-week head-to-head in obesity without diabetes, tirzepatide at its maximum tolerated dose produced roughly 22% mean body weight reduction against roughly 15% for semaglutide at its maximum tolerated dose. In SURPASS-2 in type 2 diabetes, tirzepatide 15 mg gave an HbA1c reduction of 2.46% against 1.86% for semaglutide 1 mg. Those are comparative efficacy results at each drug’s own doses. They are not conversion factors, and treating them as such would be a misreading.
What the labels say to do
Both tirzepatide labels instruct every patient to begin at 2.5 mg once weekly and increase in 2.5 mg steps at intervals of at least four weeks, to a maximum of 15 mg. Neither label carries any exception, shortcut or dose-matching provision for someone already on a GLP-1.
The only cross-product switching guidance in any of these labels is within the semaglutide family: the Wegovy label sets out moving between its injection and its tablets, in both directions. Nothing in any label bridges semaglutide to tirzepatide.
What a conversion chart would have to know
It is worth being concrete about what is missing, because “no official conversion” sounds like an administrative gap that someone could close. It is not. A dose conversion between two drugs requires a quantity that both doses can be expressed in — receptor occupancy at a shared receptor, a shared biomarker, or a shared potency unit. Between these two there is none:
- The receptor sets differ. Semaglutide acts at GLP-1. Tirzepatide acts at GIP and GLP-1. Any milligram equivalence would have to decide what a milligram of GIP activity is worth in GLP-1 terms, and nothing establishes an exchange rate.
- The escalation schedules are built differently. Semaglutide steps 0.25 → 0.5 → 1 → 1.7 → 2.4 mg; tirzepatide steps 2.5 → 5 → 7.5 → 10 → 12.5 → 15 mg. The step counts, the intervals and the ceilings do not correspond, so even a correct mapping of maintenance doses would not map the steps in between.
- The clearance timescales differ. Semaglutide’s half-life is about a week and the label describes it remaining in circulation roughly 5 to 7 weeks after a last dose; tirzepatide’s is about 5 days. Two drugs that empty at different rates cannot be swapped on a single conversion figure even if one existed.
The charts in circulation sidestep all of this by anchoring on trial weight-loss percentages and pairing doses that produced similar numbers. That is not a conversion. It is two independent trial results, in different populations, laid side by side.
What actually changes in the syringe
The part nobody writes about, and the part that matters if you are reconstituting rather than using a pen: the two drugs live at very different mass scales. Semaglutide’s approved doses top out at 2.4 mg weekly on Wegovy. Tirzepatide’s reach 15 mg.
At a shared concentration of 10 mg/mL, 2.4 mg is 24 units on a U-100 barrel and 15 mg is 150 units — more than a 1 mL barrel holds in one draw. A concentration that was comfortable for one drug will not necessarily work for the other, and the fix is a different water volume rather than a different syringe. This is an arithmetic problem rather than a dosing one, and it is the one part of switching that a calculator can actually answer. Work it out for your own vial.
What clinicians appear to do in practice
Clearly marked as non-official: the Pharmaceutical Journal reviewed this in 2025 and stated that there is currently no NICE or MHRA guidance specifying how to switch between tirzepatide and semaglutide. For the reverse direction it describes the safest approach as restarting semaglutide at its lowest available dose, 0.25 mg weekly, and titrating gradually per the standard schedule regardless of the previous tirzepatide dose.
On washout it describes clinician judgement rather than a rule — a one to two week gap preferred where there have been significant side effects, or a same-day switch for a stable patient. That is practice convention, not labelling, and we present it as such.
The practical answer
Start at the label’s starting dose and titrate on the label’s schedule. That is what both approved labels instruct, it needs no conversion factor, and it is the only answer here with a primary source behind it.
For the reconstitution side — tirzepatide and semaglutide each have their own calculator with the label’s titration schedule reproduced and cited.