Switching from Retatrutide to Tirzepatide: What the Conversion Question Gets Wrong

The biggest misconception: that a dose-conversion chart can bridge two different drugs

Retatrutide and tirzepatide are not interchangeable versions of the same medicine. Tirzepatide activates two receptors (GIP and GLP-1) slowing gastric emptying, reducing appetite and improving blood-sugar regulation. Retatrutide adds a third: the glucagon receptor. That extra pathway changes the pharmacology meaningfully, producing effects on energy expenditure that tirzepatide does not replicate in the same way. If you want to understand exactly what mixing retatrutide and tirzepatide would actually involve and why clinicians treat them as distinct molecules, that page sets out the pharmacological case clearly. Mapping one drug's milligram dose onto the other's is a bit like converting a bicycle gear ratio into a car gear ratio — the number might come out, but it describes different mechanical forces entirely.

Phase 3 trial data for retatrutide is still being gathered. Without completed safety and efficacy data across the full dose range, and without head-to-head pharmacokinetic studies comparing the two molecules, any conversion figure is an educated guess dressed up as a table. The NHS patient information for tirzepatide describes how the licensed medicine works and what its dose schedule looks like, it makes no reference to retatrutide equivalence, because none exists in any approved guidance.

If you are feeling frustrated by this answer, that is completely understandable. People researching retatrutide are often those who have read the early trial results and found them striking; the instinct to plan ahead is reasonable. The honest position is that planning around an unlicensed molecule's dose is premature.

What the trial data says about each drug separately

Tirzepatide's clinical record is extensive. In the SURMOUNT-1 trial, published in the New England Journal of Medicine, adults with obesity who took the 15 mg dose lost around 20–21% of their body weight on average over 72 weeks, a result that led NICE to recommend tirzepatide for eligible adults in its appraisal TA1026, published in December 2024. The SURMOUNT-5 head-to-head trial later showed tirzepatide produced greater average weight reduction than semaglutide 2.4 mg over the same period.

Retatrutide phase 2 data (published in the New England Journal of Medicine in 2023) showed average weight reductions up to around 24% at the highest dose over 48 weeks, which generated considerable interest. Phase 3 is ongoing. Those figures cannot be directly compared to tirzepatide's SURMOUNT results: different study designs, different durations, different patient populations, and (critically) a lower dose was titrated over a shorter period with no long-term safety follow-up yet complete.

The practical implication is that there is a licensed drug with a full safety and efficacy dossier, an NHS recommendation and a clear titration schedule available now, and an unlicensed drug that may or may not reach UK marketing authorisation in the next several years. Detailed background on how tirzepatide compares to other options is covered in our tirzepatide overview.

Why 'tirzepatide to retatrutide conversion' searches the same dead end

The reverse query (converting a tirzepatide dose into a retatrutide equivalent) runs into the same wall. Patients currently on Mounjaro sometimes ask whether they could move to retatrutide when it becomes available, and roughly what dose they would start at. The question makes intuitive sense: if you are stable on 10 mg tirzepatide, a conversion would feel useful. But a prescriber transitioning a patient between two drugs with overlapping but non-identical mechanisms would not use a milligram chart. They would take a full clinical history, consider the wash-out period, assess current side-effect profile and start at the new drug's recommended initiation dose.

That process is described in our guide to Mounjaro dosing context for readers wanting to understand how titration works in practice. The short version: dose equivalence across mechanistically distinct GLP-1 class drugs is not a solved problem even between licensed medicines. Between a licensed and an unlicensed drug, it is not a problem that has been attempted in any peer-reviewed form.

For anyone curious about what the retatrutide and Mounjaro comparison looks like from a clinical-trial perspective, this page sets out what the evidence does and does not support. And if you want to understand cost alongside clinical context, our Mounjaro cost guide explains what legitimate private treatment prices reflect and why.

The practical path forward if you are considering licensed weight-management treatment

Tirzepatide is available in the UK as Mounjaro, through a private prescription following clinical assessment. The licensed starting dose is 2.5 mg; the prescriber titrates upward based on tolerability and response, typically in four-week steps, up to a maximum of 15 mg. Eligibility under the UK licence covers adults with a BMI of 30 or above, or 27 or above alongside a weight-related condition such as hypertension, high cholesterol or type 2 diabetes. Lower BMI thresholds can apply for some ethnic backgrounds under UK guidance.

Retatrutide is not an option your prescriber can offer you today, regardless of what dose you have read about in trial reports. The medicine does not exist as a licensed product; no UK pharmacy can dispense it legally. Speculating about a tirzepatide-to-retatrutide conversion (or its reverse) does not bring that timeline forward.

If you would like to explore whether Mounjaro is clinically appropriate for you, our prescribers review consultations the same day. A real clinician reads your answers, not automated triage software. You can read more about how the treatment works and who it is licensed for on the Mounjaro page, or see the full range of weight-management options on our treatment overview. When you are ready, start your free consultation and a GPhC-registered Independent Prescriber will assess your suitability.