What Is Retatrutide and How Does It Work?

Retatrutide (LY3437943) is an investigational medication developed by Eli Lilly that represents the next frontier in GLP-1-based weight loss therapy. 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 attack obesity through multiple biological pathways at once, potentially producing greater weight loss than any currently available medication. 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 and is believed to contribute to its superior weight loss over semaglutide.

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 could explain why retatrutide has shown the highest weight loss percentages in clinical trials to date.

In a Phase 2 clinical trial published in The New England Journal of Medicine, retatrutide demonstrated unprecedented weight loss results 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 results were dose-dependent. At the 12 mg dose, participants lost an average of 24.2% of their body weight -- the highest average weight loss ever recorded for any pharmaceutical intervention in a randomized clinical trial. The 8 mg dose produced approximately 22% weight loss, and even the 4 mg dose achieved roughly 17%. By comparison, semaglutide 2.4 mg produces about 15% and tirzepatide 15 mg produces about 22.5% in their respective pivotal trials.

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.