Retatrutide (LY3437943) is currently in Phase 3 clinical trials by Eli Lilly. An FDA approval decision is anticipated in 2026-2027. This page covers research and educational context only -- this is not an approval announcement.
What it is: An investigational weekly injection engineered to simultaneously activate three hormone receptor systems - GLP-1, GIP, and glucagon - for metabolic regulation.
Research suggests: Phase 2 trials showed average body weight reductions up to 24% over 48 weeks, the highest of any weight loss compound studied in a controlled trial as of 2024.
Best for: Obesity and advanced metabolic researchers
Key thing to know: Not yet FDA approved; Phase 3 trials are ongoing as of 2025, and the long-term safety profile of triple receptor agonism is still being characterized.
What is Retatrutide?
Retatrutide is an investigational triple incretin receptor agonist , a single molecule engineered to simultaneously activate three distinct hormone receptor systems: GLP-1 (glucagon-like peptide-1), GIP (glucose-dependent insulinotropic polypeptide), and the glucagon receptor. All three of these receptors play roles in metabolic regulation, energy balance, and glucose homeostasis, but through different and potentially synergistic mechanisms. As of 2025, retatrutide is not FDA approved and is undergoing Phase 3 clinical trials.
Researchers have studied retatrutide primarily for weight loss and metabolic health optimization, with additional scientific interest in its potential role in non-alcoholic fatty liver disease (NAFLD/NASH) given glucagon's effects on hepatic fat metabolism. Phase 2 data generated significant scientific attention by demonstrating weight reductions that exceeded the results previously seen with any approved anti-obesity medication.
How it works.
Retatrutide builds on the dual mechanism of tirzepatide by adding a third pathway: glucagon receptor co-activation. GLP-1 receptor activation reduces appetite and slows gastric emptying. GIP receptor activation enhances insulin secretion and may improve fat tissue insulin sensitivity.
Together these two pathways form the foundation that already makes tirzepatide more effective than GLP-1 monotherapy.
The glucagon receptor addition is the mechanistic bet that distinguishes retatrutide. Glucagon is typically thought of as a blood-sugar-raising hormone , the physiological counterpart to insulin. But at the doses used in retatrutide research, glucagon receptor activation in the liver appears to increase energy expenditure and accelerate hepatic fat breakdown (lipolysis), potentially adding a thermogenic and fat-mobilizing component to the appetite suppression and insulin sensitization already delivered by GLP-1 and GIP.
Think of it as adding a fat-burning accelerator on top of the appetite and blood sugar regulation provided by the other two pathways. The hypothesis , currently being tested in Phase 3 , is that all three mechanisms together produce synergistic metabolic effects that exceed what any dual combination achieves. The Phase 2 weight loss data is consistent with this hypothesis, though Phase 3 confirmation across larger populations is required to establish it definitively.
What the research shows.
Phase 2 trial data published in 2023 showed retatrutide produced average body weight reductions of up to 24.2% over 48 weeks at the highest dose studied , the largest weight reduction ever reported in a randomized controlled trial of an anti-obesity medication at the time of publication. These results generated substantial scientific interest and provided compelling rationale for advancing to Phase 3 development. The dose-response relationship was clear and consistent across the trial.
Phase 2 trials are designed primarily to establish dosing ranges and preliminary efficacy signals , they are not powered or designed to provide the definitive safety and efficacy evidence that Phase 3 trials deliver. The Phase 2 data on retatrutide is therefore promising but not yet confirmatory. Phase 3 trials were initiated following these results and are ongoing as of 2025 , their completion will be critical to establishing the full safety profile, long-term efficacy, and cardiovascular outcomes for this compound.
Additional signals of interest from Phase 2 data include reductions in liver fat content , consistent with the expected hepatic effects of glucagon receptor activation , and favorable effects on lipid markers. These findings are being examined further in dedicated Phase 3 substudies.
Biomarkers to review first.
Research protocols for retatrutide typically reference the following biomarkers as baseline context. The addition of glucagon receptor agonism makes liver function monitoring particularly relevant alongside the standard metabolic panel.
What it's commonly researched with.
Retatrutide appears primarily in comparison research alongside semaglutide and tirzepatide, where investigators examine the incremental contribution of the glucagon receptor pathway to metabolic outcomes. These are comparison studies, not combination protocols , all three compounds act on overlapping pathways and are not researched in combination with each other.