What is retatrutide?

The short answer

Retatrutide (development code LY3437943) is an investigational medicine made by Eli Lilly. It is given as a subcutaneous injection once a week, and it activates three hormone receptors involved in appetite and metabolism: GIP (glucose-dependent insulinotropic polypeptide), GLP-1 (glucagon-like peptide-1) and glucagon receptors. That triple action is why researchers call it a “triple G” or triple-agonist medicine.

It is being developed for obesity, overweight with related conditions, and type 2 diabetes — and, in companion trials, for conditions driven by excess weight such as knee osteoarthritis and cardiovascular disease. No brand name has been announced. It is not approved for use anywhere, including the UK.

Why three hormones instead of one or two?

After you eat, your gut releases signalling hormones. Two of them — GLP-1 and GIP — are the basis of the medicines already familiar in the UK: semaglutide (Wegovy, and Ozempic for diabetes) mimics GLP-1 alone; tirzepatide (Mounjaro) mimics GIP and GLP-1 together. Both slow stomach emptying, increase fullness and steady blood sugar, which is why they reduce appetite and body weight.

Retatrutide adds a third signal: glucagon receptor activation. In the trial literature, the proposed contribution is that glucagon agonism increases energy expenditure and drives the liver to burn stored fat, while GLP-1 (and, to a degree, GIP activity) counters glucagon's tendency to raise blood sugar. Whether the third receptor is what produces retatrutide's larger weight-loss numbers is a reasonable inference from the data — but it remains an inference until the head-to-head trial against tirzepatide reports.

A brief history

• Phase 1 (2021–2022): first-in-human studies established the weekly dosing schedule and the step-up titration (starting low, increasing gradually) used throughout the later trials.
• Phase 2 in obesity (published June 2023, New England Journal of Medicine): the trial that made retatrutide famous. Adults with obesity lost up to an average of 24.2% of body weight at 48 weeks on the highest dose — unprecedented for a medicine at that point.
• Phase 2 in type 2 diabetes (Lancet, 2023): strong blood-sugar control plus meaningful weight loss.
• Phase 3 TRIUMPH programme (2024–2026): six-plus large trials covering obesity, obesity with diabetes, obesity with cardiovascular disease, knee osteoarthritis, and a head-to-head against tirzepatide. Five readouts had reported by July 2026 — see our trial results page.
• Next milestone: Lilly has said it plans to file a Biologics License Application with the FDA in Q1 2027. The UK pathway is covered on our timeline page.

How it would be taken — if approved

Based on the trial design, retatrutide would start at a low dose (2 mg in the Phase 2 programme) and step up every four weeks or so, precisely because the gastrointestinal side effects are dose-related and titration improves tolerability. The Phase 3 maintenance doses studied were 4 mg, 8 mg and 12 mg once weekly. All of this is investigational detail, not dosing guidance — nobody outside a trial should be taking it, and this site never provides dosing instructions for unlicensed products.

What retatrutide is not

• It is not available, for any price, in any country — anything sold as retatrutide is not genuine.
• It is not a “stronger Mounjaro” you can ask a pharmacy to source; unlicensed sourcing is illegal in the UK.
• It is not a shortcut that skips diet, movement and clinical supervision; every trial paired the medicine with lifestyle support.