At a glance

What is retatrutide?

Retatrutide is an experimental drug that activates three hormone receptors in a single molecule: GLP-1, GIP, and glucagon (sometimes called a "triple-G" agonist). It is being developed by Eli Lilly under the code LY3437943 as a once-weekly injection for obesity, type 2 diabetes, and related conditions including fatty liver disease. It is investigational and not yet approved.

How does it work?

Retatrutide combines three hormone actions. The GLP-1 and GIP receptors¹ are incretin targets whose activation enhances glucose-dependent insulin release and reduces appetite. The third target, the glucagon receptor², is the distinguishing feature: activating it is thought to increase energy expenditure and mobilize fat from the liver.

The idea is that adding glucagon action to the two incretin effects boosts weight loss and reduces liver fat beyond what incretin-only drugs achieve. This describes the proposed mechanism, which the trials below are designed to test.

Is it FDA-approved?

No. Retatrutide is investigational and not approved. According to Eli Lilly, following positive Phase 3 readouts the company plans to file with the FDA in 2027, a timeline it has attributed partly to manufacturing and quality verification.

What does the research show?

The evidence spans large, consistent trials. According to PubMed, the Phase 2 trials and the Phase 3 program reported these results:

Gastrointestinal effects were the most common side effects and were dose-related. These results are consistent across replicated trials, though the drug is not yet approved.

Frequently asked questions

Is retatrutide FDA-approved?

No. As of September 2026, retatrutide is not FDA-approved for any indication. It is an investigational compound in Phase 3 clinical development (the TRIUMPH program), Lilly plans to file for approval⁶ in the first quarter of 2027, it is not available by prescription, and it cannot be used in compounding under federal law⁷. Access is limited to authorized clinical trial enrollment.

What makes retatrutide different from tirzepatide?

Tirzepatide is a dual agonist that targets GLP-1 and GIP receptors. Retatrutide adds a third mechanism: glucagon receptor agonism. This third receptor is thought to increase energy expenditure and mobilize fat from the liver in ways that GLP-1 and GIP engagement alone do not produce. This comparison is for scientific context only; retatrutide and tirzepatide have fundamentally different regulatory statuses.

What weight loss did retatrutide produce in clinical trials?

In the 2023 Phase 2 obesity RCT⁸ published in the NEJM (n=338, 48 weeks), retatrutide at the 12 mg dose produced mean weight loss of 24.2% versus 2.1% for placebo. Individual results varied across dose groups and participants. In the Phase 3 TRIUMPH-1² trial in obesity without diabetes, mean weight loss was roughly 24-25% at the higher doses over 80 weeks.

What is the TRIUMPH trial program?

TRIUMPH is Eli Lilly's Phase 3 registrational trial program for retatrutide, consisting of four trials enrolling more than 5,800 participants. The program evaluates retatrutide for weight management (including in people with cardiovascular disease), obstructive sleep apnea, and knee osteoarthritis. The trial program design was published⁹ in Diabetes, Obesity and Metabolism in 2026. Lilly says its Phase 3 data package is now complete⁶, and TRIUMPH-1² reported mean weight loss of roughly 24-25% at the higher doses over 80 weeks. FDA approval depends on a filing Lilly plans for the first quarter of 2027 and the review that follows.

Can you get retatrutide without joining a clinical trial?

No. As of September 2026, retatrutide is not available by prescription, and it cannot be used in compounding under federal law⁷. It is an investigational compound from Eli Lilly. FDA has warned companies that illegally sold retatrutide⁷ falsely labeled "for research purposes," so any vendor offering it outside an authorized clinical trial is selling an unapproved product. The only legitimate access pathway is through authorized clinical trial enrollment; current trials can be found at ClinicalTrials.gov¹⁰.

What side effects has retatrutide shown in trials?

The most common adverse events in the Phase 2 obesity trial were gastrointestinal⁸. These events were dose-related and mostly mild to moderate in severity. In the Phase 2 obesity trial, they were less frequent in groups that started at 2 mg than in groups that started at 4 mg. The complete long-term safety profile of retatrutide has not been established; Phase 3 trials are the primary data source for this information.

How does retatrutide affect the liver?

In a Phase 2a RCT in MASLD¹¹ published in Nature Medicine in 2024 (n=98, 24 weeks), retatrutide at 8 mg and 12 mg produced 81.4% and 82.4% relative reductions in hepatic lipid content versus a 0.3% increase with placebo. These liver-fat reductions tracked with changes in body weight, abdominal fat, and metabolic measures linked to improved insulin sensitivity and lipid metabolism.

When will retatrutide be available?

A regulatory timeline cannot be confirmed. Lilly has reported topline TRIUMPH results and plans to file for FDA approval⁶ in the first quarter of 2027. As of September 2026, retatrutide is not approved, and availability would depend on the FDA's review of that application. Regulatory outcomes are not predictable from outside the process.

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