Mounjaro or retatrutide — which is actually better for weight loss?

What exactly is retatrutide, and how does it differ from Mounjaro?

Mounjaro works by activating two gut-hormone receptors: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). Both slow gastric emptying, reduce appetite, and influence how the body regulates blood sugar. You can read more about the specific mechanism on the ingredients and mechanism page for Mounjaro. Retatrutide adds a third target to that combination, the glucagon receptor. Glucagon normally raises blood glucose and plays a role in fat metabolism and energy expenditure. The theory is that stimulating all three pathways simultaneously produces greater metabolic effect than the dual approach alone.

In practice, the phase 2 retatrutide trial (published in the New England Journal of Medicine in 2023) involved 338 adults with obesity but without type 2 diabetes, treated for 48 weeks. At the highest dose tested (12mg weekly), participants lost an average of around 24% of their body weight. Those are striking numbers. They are also phase 2 numbers, a smaller, shorter trial designed primarily to identify doses and tolerability, not to confirm efficacy at the scale needed for regulatory submission. Phase 3 trials are ongoing; no results have been published. No licence application has been submitted to the MHRA.

Mounjaro, by contrast, completed a full phase 3 programme. SURMOUNT-1 randomised 2,539 adults and ran for 72 weeks, reporting around 20–21% average weight reduction at the 15mg maintenance dose. That evidence base supported NICE's recommendation of tirzepatide (NICE TA1026, published December 2024), which is the foundation of both NHS access and private prescribing in the UK today.

Is there a meaningful gap between the two medicines on weight loss results?

On raw trial percentages, retatrutide's phase 2 figures sit above Mounjaro's phase 3 numbers. Around 24% versus around 20–21% looks like a clear difference. But comparing across trial phases is genuinely unreliable. Phase 2 trials tend to recruit more tightly defined populations, run at higher dose intensities than would eventually reach market, and lack the statistical power of a 2,500-person phase 3 study. The retatrutide figure could rise, fall or hold in phase 3, nobody knows yet. You can explore how the current head-to-head evidence between tirzepatide and semaglutide was handled in SURMOUNT-5 on the retatrutide and Mounjaro comparison page.

What phase 2 data does tell us is that the triple-agonist approach is plausible and the side-effect profile was broadly similar to GLP-1 class medicines (predominantly gastrointestinal: nausea, vomiting, diarrhoea, constipation). Whether retatrutide's additional glucagon activity produces clinically meaningful advantages over Mounjaro in a real-world population (including people with type 2 diabetes, cardiovascular disease, and the full range of comorbidities) is precisely what phase 3 is designed to answer.

For anyone following this topic closely, the tirzepatide versus retatrutide comparison and the detailed retatrutide-or-tirzepatide page cover the mechanism and evidence in more depth.

Can I access retatrutide through a UK pharmacy right now?

No. Retatrutide is not approved by the MHRA, recommended by NICE, or available through any legal UK prescribing route for weight management as of summer 2026. Any seller offering it (or claiming it is equivalent to Mounjaro) is operating outside the licensed UK supply chain. The MHRA has consistently warned that weight-loss medicines sourced from unverified online sellers carry serious risks, including counterfeit products containing the wrong active ingredient. Report any suspicious seller via the MHRA Yellow Card scheme.

For people who qualify on eligibility grounds (broadly adults with a BMI of 30 or above, or 27 or above alongside a weight-related condition such as hypertension or type 2 diabetes) Mounjaro is the licensed option with the strongest current evidence base. The NICE recommendation (TA1026) sets NHS access criteria, but private prescribing through a regulated pharmacy follows the licensed indications in the SmPC. If you want to understand the clinical profile of tirzepatide in more detail, or look at how weight-loss treatment options compare more broadly, those pages are good starting points before deciding whether a consultation makes sense for you.

A quick note on what the process actually looks like: if a prescriber approves Mounjaro following your consultation, the pen arrives in a plain, unbranded box via DPD the next working day, tracked to your door. The pen itself is a pre-filled KwikPen; each one contains four once-weekly doses. It lives in the fridge, and the Patient Information Leaflet walks you through everything from the first injection to what to do if you miss one.

Information on how to obtain Mounjaro through a regulated UK prescriber and the full Mounjaro treatment overview cover the practical steps clearly. Pricing context for private treatment sits on the treatment and consultation page, a useful read before starting.