What Is Retatrutide and How Does It Work?

Retatrutide (LY3437943) is an investigational medication developed by Eli Lilly. Unlike semaglutide, which targets a single receptor (GLP-1), or tirzepatide, which targets two receptors (GLP-1 and GIP), retatrutide activates three receptors simultaneously: GLP-1, GIP, and glucagon.

This triple-agonist approach is designed to act on obesity through multiple biological pathways at once. Retatrutide is not yet FDA-approved and is currently in Phase 2 and Phase 3 clinical trials. It is not available through pharmacies, compounding facilities, or any legal commercial channel.

Each of the three receptors targeted by retatrutide plays a distinct role in metabolism and weight regulation:

GLP-1 (glucagon-like peptide-1) receptor: Reduces appetite by acting on hunger centers in the brain, slows gastric emptying to increase fullness, and improves blood sugar control by stimulating glucose-dependent insulin secretion. This is the same receptor targeted by semaglutide and one of the two targeted by tirzepatide.

GIP (glucose-dependent insulinotropic polypeptide) receptor: Enhances insulin sensitivity, supports fat metabolism, and may amplify the appetite-suppressing effects of GLP-1 signaling. GIP receptor agonism is the second target in tirzepatide.

Glucagon receptor: This is what makes retatrutide unique. Glucagon activation increases energy expenditure by promoting thermogenesis (calorie burning), stimulates the liver to mobilize stored fat for energy, and may reduce liver fat accumulation. This additional mechanism is the basis for the triple-agonist approach studied in retatrutide clinical trials.

In a Phase 2 clinical trial published in The New England Journal of Medicine, retatrutide was studied across all dose groups. Participants received weekly subcutaneous injections at doses of 1 mg, 2 mg, 4 mg, 8 mg, or 12 mg over 48 weeks.

The trial used a gradual escalation protocol for the higher-dose groups to improve tolerability, similar to the approach used with other GLP-1-class medications. Each dose group followed a structured titration schedule before reaching its target maintenance dose.

Gastrointestinal side effects were the most common adverse events, consistent with the GLP-1 class. The safety profile was generally similar to existing GLP-1 medications, though larger Phase 3 trials are needed to fully characterize long-term safety.

As of 2025, retatrutide is in Phase 3 clinical trials being conducted by Eli Lilly. These larger trials will evaluate the medication's efficacy and safety in broader populations and are required for FDA submission. If the Phase 3 results are consistent with Phase 2 data and no significant safety concerns emerge, Eli Lilly could submit a New Drug Application (NDA) to the FDA.

The timeline for potential FDA approval remains uncertain but optimistic estimates suggest it could be several years away. Until then, retatrutide is not available for prescription, purchase, or compounding. Any source claiming to sell retatrutide for patient use outside of a clinical trial is operating outside legal and regulatory boundaries.