Retatrutide vs Wegovy: what the trial data actually tells us

What the phase 2 retatrutide data showed — and why caution is still warranted

Retatrutide is an investigational triple agonist made by Eli Lilly, activating GIP, GLP-1 and glucagon receptors simultaneously. A phase 2 trial published in the New England Journal of Medicine (2023) randomised 338 adults with obesity (without diabetes) to different doses of retatrutide or placebo over 48 weeks. At the highest dose tested (12mg), participants lost an average of around 17.5% of body weight, in 48 weeks, which is a notably shorter period than most phase 3 obesity trials run. Some analyses put the trajectory higher if extrapolated to 72 weeks, though that comparison requires care: different trial durations, populations and endpoints make direct arithmetic comparisons unreliable.

Phase 3 trials are currently underway. Until those results are published and reviewed by regulators, the phase 2 figure is the only published clinical evidence available. Phase 2 trials are designed to assess dose and tolerability in smaller populations; they are not the basis on which regulators license medicines. Retatrutide has not been submitted to the MHRA, and no timeline for a UK submission has been publicly confirmed.

That context matters. The NHS medicines page for semaglutide covers Wegovy's licensed status clearly, and the NICE appraisal TA875 sets out the full evidence base that supported its approval, a 68-week phase 3 trial in over 1,900 participants. The bar for licensing is deliberately high.

How Wegovy's documented results compare on a like-for-like basis

Wegovy's STEP 1 trial (68 weeks, 1,961 adults, phase 3) recorded an average body-weight reduction of around 15% at the 2.4mg weekly maintenance dose. Following MHRA approval of a 7.2mg dose (confirmed for the single-dose pen in April 2026), later data reported average weight loss of roughly 20.7% over 72 weeks at that higher dose, narrowing the gap with tirzepatide trial results considerably. These are phase 3 figures in large populations, the regulatory standard.

The table below sets out what is verifiably known about each medicine at this point, and our page on the difference between retatrutide and semaglutide goes into greater detail on how the two molecules diverge mechanistically. Numbers in the retatrutide column reflect phase 2 only; treat them as indicative, not definitive.

One practical check worth doing in under a minute: visit the GPhC pharmacy register and search for any online pharmacy before handing over personal or payment details. A registered pharmacy will appear immediately. If it does not, that is a serious warning sign regardless of what the website claims.

Side-effect profiles: what each mechanism suggests

Wegovy's side-effect profile is well-characterised across large phase 3 populations. The most common effects are gastrointestinal (nausea, vomiting, diarrhoea, constipation, reflux) typically most noticeable in the first few weeks or after a dose step, then settling for most people. In January 2026 the MHRA issued a Drug Safety Update drawing attention to acute pancreatitis as an infrequent but potentially serious effect across GLP-1 medicines: severe stomach pain that spreads to the back, with or without vomiting, warrants urgent medical attention. You can report suspected side effects at yellowcard.mhra.gov.uk.

Retatrutide's phase 2 trial reported a broadly similar GI-led side-effect pattern. The glucagon component is thought to increase energy expenditure, which some researchers believe may contribute to the weight-loss signal, but it also raises questions about lean-mass preservation that phase 3 data will need to address. Anyone wondering whether retatrutide is better than semaglutide should bear in mind that without large-scale, long-duration safety data, no fair comparison of side-effect burden between the two medicines is yet possible.

If you want to explore the wider picture of how semaglutide compares across its different presentations, the Wegovy vs Ozempic page explains why the same molecule carries different licences depending on its dose and brand. And for a closer look at what switching might look like once retatrutide reaches UK approval, our page on whether you can switch from semaglutide to retatrutide covers what is currently known.

What this means for someone looking for weight-loss treatment in the UK today

If you are searching for retatrutide in the UK, the honest answer is that it is not available, from any registered pharmacy, because it is not licensed. Sellers offering it are operating outside the law, and any pen labelled as retatrutide from an unverified source carries the same counterfeit risks that prompted the MHRA to warn repeatedly about fake weight-loss pens circulating online. For a thorough look at how the two medicines stack up on the evidence published so far, our retatrutide vs semaglutide comparison page brings together the key trial data, though large quantities of illegally traded weight-loss medicines have been seized in UK enforcement operations since 2023 and no reputable source can supply retatrutide today.

Wegovy is available now through a private prescription, following clinical assessment. It has a large, well-documented evidence base, a defined dosing pathway, and an established side-effect profile that prescribers understand. Our team at nume reviews consultations the same day (a named prescriber reads your answers) and if Wegovy is clinically suitable, treatment can be dispatched the same day you order before midday. You can see the full range of licensed weight-loss options on our weight-loss treatment overview.

Retatrutide may well reach the UK market after full phase 3 data and regulatory review. That day is not today. For patients who are already on semaglutide and want to understand what a future move might involve, our guide on switching from semaglutide to retatrutide sets out what the current evidence suggests about that process. Which licensed treatment suits you is a clinical decision our prescribers work through with you during the consultation, not a choice to make from a trial abstract alone.

Start your free consultation and find out which option is right for you.