Investigational, not approvedInvestigational GIP, GLP-1 and glucagon receptor triple agonist (peptide)

Retatrutide

Also known as LY3437943, reta

Retatrutide (LY3437943) is an investigational peptide from Eli Lilly that activates three hormone receptors: GIP, GLP-1 and glucagon. It is in Phase 3 trials and is not approved anywhere.

By [Author name]. Licensed pharmacist review: pending. Last updated 2026-10-21.

Identity

Developer: Eli Lilly and Company
Class: GIP / GLP-1 / glucagon receptor triple agonist
Structure: 39-amino-acid peptide with a fatty-acid side chain for albumin binding
Molecular formula: C221H342N46O68 (PubChem)
Molar mass: about 4,731 g/mol
Half-life: About 6 days in Phase 1 pharmacokinetic studies, which supports once-weekly dosing in trials.

What the research covers

Retatrutide was designed to add glucagon-receptor activity to the GIP and GLP-1 activity of earlier incretin drugs. Glucagon signalling affects energy expenditure and liver fat, which is the rationale researchers give for the third target.

A Phase 2 trial in adults with obesity (published in the New England Journal of Medicine in 2023) ran for 48 weeks and measured change in body weight against placebo. A separate Phase 2 trial in adults with type 2 diabetes (The Lancet, 2023) measured glycated hemoglobin and body weight.

Lilly's Phase 3 program, TRIUMPH, is testing retatrutide in obesity, type 2 diabetes, knee osteoarthritis, sleep apnea and cardiovascular and kidney outcomes. Gastrointestinal effects (nausea, diarrhea, vomiting) and heart-rate increases were the most commonly reported adverse events in Phase 2.

Summaries describe what studies measured, not what a substance does for anyone. Nothing here is a claim of benefit or a basis for use.

Regulatory status

Not approved by FDA or any other regulator as of the review date.
Cannot be legally compounded in the US: it is not an approved drug, has no USP monograph and is not on FDA's 503A bulk substances list.
FDA has sent warning letters to companies selling retatrutide labeled "for research use only", including the August 24, 2026 letters, which cited retatrutide by name.

Anti-doping (WADA)

Not specifically named, but WADA's S0 class bans any pharmacological substance not approved for human therapeutic use, which covers investigational drugs like retatrutide.

Follow changes on the FDA peptide status tracker.

Retatrutide questions

Is retatrutide the same as tirzepatide?

No. Tirzepatide activates two receptors (GIP and GLP-1) and is FDA-approved as Mounjaro and Zepbound. Retatrutide adds a third target, the glucagon receptor, and is still investigational.

When will retatrutide be approved?

Lilly has not announced an approval date. Approval would follow completion of the Phase 3 TRIUMPH trials and an FDA review. The FDA status tracker on this site is updated when that changes.