Retatrutide, code LY3437943, is an experimental peptide from Eli Lilly that acts on the GIP receptor, the GLP-1 receptor and the glucagon receptor at the same time.1 It carries a fatty acid side chain that binds to albumin in the blood, which is what lets it be given once a week.
It is still in development. Phase 3 trials reported during 2026 and Lilly has said it plans to file for US approval in early 2027.6 Until a regulator approves it, it is an unlicensed investigational drug everywhere, and Lilly has sued sellers of it.
39
≈ 4731 g/mol
Up to 28% at 80 wk
Nowhere yet
Identity
Retatrutide is built on the glucagon family of peptides. Its sequence is modified at both ends to stop enzymes cutting it, and a lysine side chain carries a C20 diacid through a short linker. That lipid is what makes it last.1
| Value | |
|---|---|
| Developer | Eli Lilly and Company |
| Code | LY3437943 |
| Receptors | GIP, GLP-1, glucagon |
| Formula | C221H342N46O68 |
| Avg mass | About 4731 g/mol, calculated from the formula |
| CAS | 2381089-83-2 |
| Dosing in trials | Once-weekly subcutaneous injection |
History
Discovery and phase 1 paper published.1
Phase 2 in obesity (48 weeks) and in type 2 diabetes published.2,3
Phase 3 TRIUMPH programme begins.
TRIUMPH-4 in knee osteoarthritis reports 28.7% weight loss at 68 weeks on 12 mg.5
TRIUMPH-1 reports up to 28% weight loss at 80 weeks and about 30% at 104 weeks in an extension group.4,5
TRIUMPH-2 and TRIUMPH-3 report. Lilly says it plans a US filing in the first quarter of 2027.6
Lilly sues six companies over sales of the compound.7
What the trials measured
The primary measure in every trial is change in body weight. Secondary measures include glucose control, blood pressure, lipids and liver fat. Figures below are from the published paper for phase 2, and from Lilly's announcements and trade press for phase 3, since the phase 3 papers were not yet published at this revision.
| Trial | People | Duration | Weight change | Source |
|---|---|---|---|---|
| Phase 2, obesity | 338 adults without diabetes | 48 weeks | −8.7% (1 mg), −17.1% (4 mg), −22.8% (8 mg), −24.2% (12 mg); placebo −2.1% | [2] |
| TRIUMPH-1, obesity | 2,339 adults | 80 weeks | Up to 28.3% on 12 mg by one analysis, 25.0% by another; 30.3% at 104 weeks in a 532-person extension | [4],[5] |
| TRIUMPH-2, obesity with T2D | 1,152 adults | 80 weeks | −12.7% (4 mg), −19.1% (9 mg), −20.8% (12 mg); placebo −4.0% | [6] |
| TRIUMPH-3, obesity with CVD | 1,949 adults | 80 weeks | −21.6% (9 mg), −22.6% (12 mg); placebo −3.2%. Cardiovascular events not significantly different | [6] |
| TRIUMPH-4, knee OA | Adults with obesity and knee OA | 68 weeks | −28.7% on 12 mg | [5] |
Side effects reported are those of the drug class: nausea, diarrhoea, constipation and vomiting, more often at higher doses. In TRIUMPH-1, 4.1% to 11.3% stopped for adverse events against 4.9% on placebo.4
Regulatory and sport status
As of this revision. This is moving fast; the date on the entry matters.
| Where | Status |
|---|---|
| US | Not approved. Lilly has said it plans to file in the first quarter of 2027.6 |
| UK and EU | Not approved. No application reported. |
| Sport | Not named on the WADA list. Because it has no approval for human use, it falls under class S0, non-approved substances, prohibited at all times.8 |
| Legal | Lilly filed suit in August 2026 against six companies selling the compound.7 In the UK, the MHRA has raided operations selling unlicensed tirzepatide and retatrutide, in October 2025 and February 2026.9,10 |
Open questions
- The phase 3 papers. Everything after 2023 comes from announcements and conference data, not peer-reviewed publications.
- What the right 80-week number is. Two analyses of TRIUMPH-1 give 25.0% and 28.3% for the same dose, depending on how people who stopped are counted.4,5
- Heart outcomes. TRIUMPH-3 did not show a significant reduction in cardiovascular events over 80 weeks.6
- What happens to weight after stopping, which no trial has yet reported in full.
Common questions
Is retatrutide approved anywhere?
No. It is an investigational drug. Lilly has said it plans to file for US approval in early 2027.
How is it different from semaglutide and tirzepatide?
Semaglutide acts on one receptor (GLP-1). Tirzepatide acts on two (GLP-1 and GIP). Retatrutide acts on three, adding the glucagon receptor.
How much weight loss did the trials show?
In the phase 2 paper, 24.2% at 48 weeks on the highest dose. In TRIUMPH-1, up to about 28% at 80 weeks and about 30% at 104 weeks in an extension group, by Lilly's announcements.
Is it banned in sport?
It is not named on the WADA list, but as an unapproved substance it falls under class S0, prohibited at all times.
What is GLP-3?
GLP-3 is a trade name some suppliers use for retatrutide, after the three receptors it acts on. It is not a natural hormone. Peptidarium sells retatrutide as a research material under the name GLP-3, for laboratory use only. Lilly owns the compound and has sued some sellers, so we review our position as the legal picture changes.
See also
References
- Coskun T, et al. LY3437943, a novel triple glucagon, GIP, and GLP-1 receptor agonist for glycemic control and weight loss: from discovery to clinical proof of concept. Cell Metab. 2022;34(9):1234–1247. PMID 35985340
- Jastreboff AM, et al. Triple-hormone-receptor agonist retatrutide for obesity: a phase 2 trial. N Engl J Med. 2023;389(6):514–526. PMID 37366315
- Rosenstock J, et al. Retatrutide, a GIP, GLP-1 and glucagon receptor agonist, for people with type 2 diabetes: a randomised, double-blind, placebo and active-controlled, parallel-group, phase 2 trial. Lancet. 2023.
- AJMC. Retatrutide achieves up to 30.3% average weight loss in phase 3 TRIUMPH-1 trial. 21 May 2026. ajmc.com
- The Pharmaceutical Journal. Phase III retatrutide study demonstrates 30% weight loss. 22 May 2026. pharmaceutical-journal.com
- Pharmaceutical Executive. Retatrutide delivers up to 22.6% weight loss in two new phase III trials. 23 July 2026. pharmexec.com
- Reuters. Lilly sues six companies over sales of retatrutide. 12 August 2026.
- World Anti-Doping Agency. The 2026 Prohibited List, class S0. wada-ama.org
- Reuters. UK seizes record haul of illegal weight-loss drugs in factory raid. 24 October 2025.
- The Pharmacist. Almost 2,000 unauthorised weight loss drugs seized in MHRA raids. 26 February 2026. thepharmacist.co.uk
Peptidarium Library. Retatrutide (entry 0005), revision 1.0, 2 November 2026. peptidarium.com/library/retatrutide
Revision history
| Rev | Date | Change | By |
|---|---|---|---|
| 1.0 | 2 Nov 2026 | Entry created. | [Scientific editor] |
Spotted an error or a missing paper? Send a correction. Every change is logged here.