Mounjaro®
Starting from £179.99/mo
Start journey Learn moreWhat the evidence actually shows: trial data, mechanisms and where each medicine stands today
Two very different receptor targets, and why that gap matters clinically
Mounjaro activates two receptors: GLP-1 (glucagon-like peptide-1) and GIP (glucose-dependent insulinotropic polypeptide). That dual action is what makes it distinct from semaglutide-based medicines, which work on GLP-1 alone. Retatrutide goes further still. It is a triple agonist, adding glucagon receptor activation to the same GLP-1 and GIP pathways that tirzepatide targets. The glucagon component is designed to increase resting energy expenditure, in simple terms, it may help the body burn more calories at rest, on top of reducing appetite.
This is where a common misconception is worth addressing gently: some readers assume retatrutide is simply a stronger version of Mounjaro, as if it is the next dose up. It is not. It is a chemically distinct compound with a third mechanism not present in tirzepatide at all, and if you are wondering what the difference between Mounjaro and tirzepatide actually is, that question is worth answering before comparing either to retatrutide. Whether that third pathway translates into meaningfully better outcomes for real patients, at a tolerability profile that regulators accept, is precisely what the ongoing trials are designed to establish.
For a deeper look at how tirzepatide's dual pathway compares to single-receptor GLP-1 medicines, the GLP-1 versus tirzepatide comparison on this site covers that ground in full.
What the phase 2 trial showed, and what it does not yet tell us
A phase 2 trial of retatrutide published in The Lancet in 2023 (NEJM correspondence also covers early data) reported average weight reductions of up to around 24% over 48 weeks at the highest doses tested, in adults with obesity. That figure attracted significant attention. For context, SURMOUNT-1 (the pivotal phase 3 trial underpinning Mounjaro's licence) reported around 20–21% average weight loss at 15mg over 72 weeks, as cited in the NEJM publication of SURMOUNT-1. The retatrutide number is higher, but direct comparison is complicated: different trial lengths, different populations, different dose-finding stages, and phase 2 studies are not designed to confirm efficacy the way phase 3 studies are.
Phase 3 trials for retatrutide are underway as of mid-2026, but no results have been published, no marketing authorisation has been submitted to the MHRA, and no regulatory timeline has been announced for the UK. The MHRA licenses medicines based on the full phase 3 data package, a process that takes years after trial completion. Retatrutide is not on the horizon as a prescribable treatment in the near term.
The retatrutide versus Mounjaro page explores the trial comparisons in more detail for readers wanting the deeper numbers.
Head-to-head at a glance
| Factor | Mounjaro (tirzepatide) | Retatrutide |
|---|---|---|
| Receptor targets | GLP-1 + GIP (dual agonist) | GLP-1 + GIP + glucagon (triple agonist) |
| UK regulatory status | MHRA-licensed for weight management | Not licensed anywhere; phase 3 trials ongoing |
| Phase 3 trial weight-loss data | ~20–21% average at 15mg / 72 weeks (SURMOUNT-1, NEJM) | No phase 3 data published yet |
| Phase 2 signal | Confirmed efficacy underpinning licence | Up to ~24% at highest dose / 48 weeks (exploratory only |
| Available on private prescription in UK | Yes, subject to clinical assessment | No |
| NICE recommendation | Yes) TA1026, published December 2024 (updated September 2025) | Not applicable |
For context on how Mounjaro's evidence was assessed by the UK's clinical advisors, NICE technology appraisal TA1026 sets out the full recommendation and eligibility criteria.
What this means if you are looking for treatment now
Retatrutide is not a treatment option. It is a research compound, and no UK pharmacy can supply it, the question of which is "better" is currently unanswerable in any clinically meaningful sense, because one has not completed the trials that would let regulators make that judgement.
Mounjaro is the strongest-evidenced, MHRA-licensed option available in the UK for weight management today. The Mounjaro overview page covers the full licence detail, how the KwikPen works, and what the clinical programme found. If you want to compare what is actually available on prescription, the retatrutide vs Mounjaro analysis and the mechanism-level breakdown are both worth reading alongside this page.
Which medicine suits you is a clinical decision made with a prescriber who knows your full medical picture. At nume, a GPhC-registered Independent Prescriber reads your consultation personally. You can also explore our weight-loss treatment overview or go straight to check your eligibility, there is no obligation and the consultation is free.
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