Retatrutide or Mounjaro: which is better for weight loss?

Step 1: understand what each medicine is, and where it stands

Mounjaro is tirzepatide, made by Eli Lilly. It works on two receptors (GIP and GLP-1) both involved in regulating appetite, insulin secretion and gastric emptying. The UK's Medicines and Healthcare products Regulatory Agency has licensed it for weight management in adults with a BMI of 30 or above, or 27 or above alongside at least one weight-related condition. It has been prescribed in the UK since 2023 and its clinical programme spans thousands of adult participants across the SURMOUNT trials.

Retatrutide is also being developed by Eli Lilly, but it adds a third target: the glucagon receptor. Glucagon stimulates energy expenditure and fat breakdown, so activating it alongside GIP and GLP-1 is the scientific rationale for a potentially larger effect. As of mid-2026, retatrutide has no licence in the UK, the US or the EU. It is progressing through phase 3 trials. Anyone describing it as a treatment option they can prescribe or supply in the UK today is not being accurate. For a fuller look at how the two are framed side by side in the published literature, the retatrutide versus tirzepatide comparison goes deeper into the mechanism differences.

That distinction matters enormously before you weigh up which is "better." Licensed and unlicensed medicines exist in entirely different practical realities.

Step 2: read the trial data carefully, phase 2 is not the same as phase 3

The phase 2 retatrutide data, published in the New England Journal of Medicine in 2023, is genuinely impressive. Over 48 weeks, participants receiving the highest tested dose saw average body-weight reductions of around 24%. By comparison, the SURMOUNT-1 trial (a full phase 3 study with 2,539 participants) reported average reductions of roughly 20–21% at 15mg tirzepatide over 72 weeks, with some analyses approaching 22.5%. The numbers lean toward retatrutide at first glance.

But phase 2 trials are designed to find the right dose and check for safety signals, not to confirm efficacy at scale. They enrol far fewer participants and run for shorter periods than the pivotal phase 3 studies that regulators use to assess a medicine. The retatrutide figure comes from a study of a few hundred people over less than a year; Mounjaro's 20–21% comes from nearly 2,600 people followed for 72 weeks under conditions that regulators examined in detail before granting a licence. Treating the two sets of numbers as a straight head-to-head comparison misreads what clinical evidence stages mean. Our retatrutide versus Mounjaro page works through the trial methodology in more detail.

NICE's appraisal of tirzepatide (Technology Appraisal TA1026, published December 2024) gives a sense of the regulatory threshold a medicine must clear before it can be recommended for routine clinical use in England. Retatrutide has not yet cleared that bar anywhere.

Step 3: think about what "better" actually means for you right now

If the question is which medicine produces the larger number in a trial, retatrutide's phase 2 data points toward a higher ceiling, though that may shift when larger studies report. If the question is which medicine a prescriber can legally and safely offer you in the UK today, the answer is Mounjaro (or Wegovy, the licensed semaglutide option for weight management). Retatrutide is not on that list.

There is also the practical side. Mounjaro is dispensed as a once-weekly KwikPen injection. At a licensed service like ours at nume, the pen arrives via DPD the next working day in plain, unmarked packaging, there's nothing on the outside to suggest what's inside. Retatrutide, even once licensed, would need to establish its own supply chain, prescribing infrastructure and post-marketing safety record before it could be offered with the same confidence. For people weighing up what's available now, the Mounjaro overview covers what a UK private prescription actually looks like from eligibility through to treatment.

For those thinking about cost as part of the decision, the cost context for Mounjaro in the UK explains how private pricing works and what a legitimate prescription should include. And if Mounjaro turns out not to be the right fit, there are other licensed alternatives worth discussing with a prescriber.

Step 4: where retatrutide goes from here

Phase 3 TRIUMPH trials for retatrutide are underway. If results hold, and if Eli Lilly submits and secures regulatory approval from the MHRA, retatrutide could eventually reach UK patients, possibly in the latter half of the decade. That is a meaningful prospect, but it is not a near-term one, and licensing timelines are not guaranteed.

For people asking whether retatrutide is better than Mounjaro as a practical matter, the honest answer is: we don't know yet, and it's not available either way. What we have is a licensed, well-studied, regulated medicine in Mounjaro that a prescriber can assess you for today. A question our prescribers hear fairly often is whether it's worth waiting for something newer. The clinical answer is almost always that a medicine that exists (and has been through full regulatory scrutiny) is more useful than one that might arrive in a few years. Whether retatrutide eventually earns a place alongside or above tirzepatide will be a conversation for when the phase 3 data lands and a licence follows. For now, checking your eligibility for what's available is where that conversation starts.