Retatrutide (LY3437943)

Retatrutide is an investigational 39-amino acid peptide from Eli Lilly that activates three incretin-related receptors: GLP-1, GIP, and glucagon. The addition of glucagon agonism is the novel piece — glucagon, given at sufficient doses, increases energy expenditure, which may explain the larger weight loss seen in trials vs. dual agonists.^[1]

As of early 2026, retatrutide is in Phase 3 (TRIUMPH program) and not yet FDA-approved.

Retatrutide grew out of years of co-agonist research aimed at adding glucagon-receptor activation to the GIP/GLP-1 dual mechanism pioneered by tirzepatide. The hypothesis: glucagon, properly balanced against GLP-1's glucose-lowering, would contribute additional weight loss through increased energy expenditure rather than further appetite suppression alone. The molecule was first published in 2022 (Coskun et al.) and entered Phase 2 with reports that drew immediate attention for the magnitude of weight loss observed.

The three receptor arms contribute complementary effects:

• GLP-1 — appetite suppression, delayed gastric emptying, glucose-dependent insulin release.
• GIP — as in tirzepatide, likely modifies adipose metabolism and enhances central weight loss signaling.
• Glucagon — increases basal energy expenditure, mobilizes hepatic glycogen and triglycerides, promotes lipolysis. This is the component most strongly implicated in the additional weight loss.

Balancing glucagon's glucose-raising and its energy-expenditure effects against GLP-1's glucose-lowering is a core design challenge — the engineered receptor affinities appear to achieve net glucose control while preserving the energy-expenditure benefit.^[2]

Retatrutide is given once weekly by subcutaneous injection. Its pharmacokinetic profile is similar in structure to tirzepatide and semaglutide — a peptide backbone modified for albumin binding to extend half-life into the multi-day range. The Phase 2 dose escalation reached 12 mg weekly; the Phase 3 program is exploring doses up to 12 mg.

The peer-reviewed literature on Retatrutide is summarized below across two tiers: human research (the highest standard), and preclinical / emerging research (animal models and early-stage human work).

This table summarizes commonly discussed claims and how the published evidence weighs in. The aim is clarity — supported claims, claims that look promising but need more data, and claims that outrun the science.

In Phase 2 trials, retatrutide was administered as a once-weekly subcutaneous injection with stepwise titration to doses up to 12 mg weekly. Final approved dosing will depend on Phase 3 outcomes.

Retatrutide is investigational and not available through legitimate clinical channels outside of trials.

Retatrutide is the most exciting near-term entrant in the metabolic peptide space. Phase 2 data is remarkable, but history suggests Phase 3 results will temper expectations at least somewhat. Until TRIUMPH reads out, treat the 24% weight-loss figure as a preliminary signal rather than an established effect. Any non-trial use (grey market) carries serious identity, purity, and safety risks that go beyond the usual caveats.