Retatrutide: what it is, and what the research actually shows

What retatrutide is

Retatrutide (LY3437943) is a triple receptor agonist developed by Eli Lilly. Where semaglutide targets one receptor (GLP-1R) and tirzepatide targets two (GIP-R and GLP-1R), retatrutide adds a third: the glucagon receptor (GCGR). This triple mechanism is what makes it a distinct class of compound from the dual agonists, and it's the basis for the research interest in whether adding glucagon receptor activity produces further improvements in metabolic outcomes.

Glucagon is typically associated with raising blood glucose, which is the opposite of what you want in a diabetes or obesity treatment. That apparent paradox is resolved by the specific balance of agonism: glucagon receptor activation at the doses and formulations studied appears to increase energy expenditure and improve liver fat reduction in ways that complement GIP and GLP-1 signalling, without the hyperglycaemic effect you'd see with straight glucagon administration. The balance point matters, and getting it right at the molecular level is one of the technical challenges of triple agonist development.

Retatrutide is still investigational as of 2026. Phase 3 trials are ongoing. It has no regulatory approval anywhere in the world. The Phase 2 data is what's available in the public research literature.

What the Phase 2 trial found

The Phase 2 trial of retatrutide was published in the New England Journal of Medicine in June 2023 (Jastreboff et al., "Triple-Hormone-Receptor Agonist Retatrutide for Obesity"). It enrolled 338 participants with a BMI of 27 or above without type 2 diabetes, randomised to placebo or one of five retatrutide doses across 24 and 48 weeks.

At 48 weeks, the 12mg dose produced a mean weight reduction of 24.2% from baseline versus 2.1% for placebo. That figure was the largest mean reduction reported in any published weight management trial at the time of publication. The mechanism of body weight reduction included reductions in fat mass confirmed by DEXA scan, with preservation of lean mass at the doses studied.

The adverse event profile was similar to other GLP-1 class compounds: gastrointestinal effects (nausea, vomiting, diarrhoea) were the most common, with a dose-dependent pattern. These were generally described as mild to moderate in intensity and decreasing over time.

Phase 2 data has real limitations. A 338-person trial over 48 weeks is not the same as the thousands-of-participants, 72-week Phase 3 trials that established the tirzepatide and semaglutide evidence base. The Phase 2 results set the basis for Phase 3 investigation, not a conclusion. Phase 3 results are expected in 2025-2026, and those will determine whether the Phase 2 effect size is replicated at scale.

Why adding glucagon receptor activity matters

GLP-1 and GIP receptor agonism reduce caloric intake through appetite suppression and slowed gastric emptying. Adding glucagon receptor agonism appears to increase energy expenditure, which is a different lever. Glucagon receptor activation in the liver promotes fat oxidation and may reduce hepatic steatosis (liver fat). In the context of obesity research, which frequently involves fatty liver disease as a comorbidity, a compound that addresses both energy balance and liver fat has potential research interest beyond weight management alone.

The glucagon component also raises the risk profile question: prolonged glucagon receptor activation has theoretical effects on bone metabolism, cardiac function, and blood glucose regulation that researchers need to assess over longer trial periods. This is one reason Phase 3 trials are needed before the safety profile can be characterised as fully as it is for the approved dual and single agonists.

Development status as of 2026

Retatrutide Phase 3 trials are ongoing. The compound has no marketing authorisation from the FDA, EMA, or MHRA, and no approval timeline has been publicly confirmed for the UK. It's an investigational compound in the clinical development sense, not a compound that has passed the full regulatory review that semaglutide and tirzepatide have completed. The research compound sold here is supplied for laboratory research purposes only, not for clinical application.

Retatrutide in our catalogue

RT-5

Retatrutide, 5mg

Supplied as a lyophilised vial for laboratory research use.

£84.99 Contact us to order

RT-10

Retatrutide, 10mg

Supplied as a lyophilised vial for laboratory research use.

£114.99 Contact us to order

See our documentation policy for what supplier batch documentation covers, and our UK legal status page for the regulatory framing every listing follows.

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