Weight Loss: Retatrutide (GLP-1/GIP/Glucagon Triple Agonist)
A New Era in Triple-Agonist Therapy
In the rapidly advancing field of metabolic medicine, Retatrutide (LY3437943) marks the most recent and most potent milestone. While the current generation (such as semaglutide or tirzepatide) targets one or two hormonal pathways, Retatrutide is revolutionizing the therapeutic landscape as the first triple agonist.
📋 Summary for the Quick Reader
- Triple Agonist: Retatrutide simultaneously activates GLP-1, GIP and glucagon receptors — a novel mechanism in obesity therapy.
- Phase 2 Weight Loss: In the NEJM Phase 2 trial (Jastreboff et al, 2023, N=338), the 12 mg dose group achieved −24.2% body weight after 48 weeks (vs. −2.1% on placebo) — the highest published value among incretin mimetics.
- Phase 3 Status: The TRIUMPH program (Phase 3) for obesity, sleep apnea and knee osteoarthritis is ongoing; the TRANSCEND-T2D-1 trial for type 2 diabetes was published in The Lancet in June 2026.
- Preclinical MASH Data: In a mouse model, Retatrutide produced marked reduction of steatohepatitis. Dedicated Phase 2 trials in humans for MASH/MASLD have not yet been published.
Record-Breaking Trial Results
The published top-line data come from the Phase 2 trial by Jastreboff et al. (NEJM 2023, N=338, 48 weeks): the 12 mg dose group achieved an average weight loss of −24.2% — a new record among incretin mimetics. The frequently quoted "28.7%" figures stem from presentations of ongoing Phase 3 trials and have not yet been peer-reviewed and published.
Mechanism of Action: Three Targets, One Molecule
Retatrutide's uniqueness lies in the simultaneous activation of three critical hormone receptors:
- GLP-1 (Glucagon-like Peptide-1): Controls appetite and slows gastric emptying.
- GIP (Glucose-dependent Insulinotropic Polypeptide): Improves insulin sensitivity and fat metabolism.
- Glucagon: The "game changer" in this combination. It directly increases energy expenditure and promotes the breakdown of fat stores in the liver.
Preclinical Data on MASH (Fatty Liver)
A published animal study (December 2025) shows that Retatrutide significantly ameliorated accelerated diet-induced steatohepatitis in mice — including reductions in liver fat, inflammatory markers and fibrosis scores. Human data specifically for MASH/MASLD are still pending: in the ongoing Phase 3 trials of the TRIUMPH program (obesity, OSA, knee OA), MASH is not a primary endpoint. Early hints at hepatic effects in humans come from body-composition substudies (Coskun et al, Lancet Diabetes Endocrinol 2025), which report reductions in total body fat in T2D patients, not primarily liver fat.
Phase 3 Status and Outlook
The clinical development program for Retatrutide comprises two major Phase 3 initiatives:
- TRIUMPH Program (rationale & design published January 2026): Phase 3 basket trial for obesity, obstructive sleep apnea and knee osteoarthritis — results are still pending.
- TRANSCEND-T2D-1 (Lancet, June 2026, N=537): Phase 3 trial for type 2 diabetes, 40 weeks. HbA1c reduction −1.69 to −1.94% (vs. −0.81% placebo), weight loss −11.5 to −15.3%.
The side-effect profile largely mirrors other incretin mimetics (nausea, diarrhea), with dose titration being critical for tolerability. Concrete approval timelines for the FDA or EMA have not been officially communicated as of June 2026.
Bottom Line
Retatrutide represents a significant advance in the pharmacological treatment of obesity and metabolic disorders. As the first triple agonist, it combines three mechanisms of action and shows impressive weight-loss results in clinical trials.
The Phase 2 data are promising, but Phase 3 evidence — particularly regarding cardiovascular outcomes and long-term safety — remains to be seen. For patients with obesity who do not adequately respond to conventional therapies, Retatrutide could become an important therapeutic option.
Important note: Retatrutide is an investigational drug and is currently not approved. Patients should inquire about clinical trials and should not self-medicate.
💉 Study Dosages (Retatrutide / LY3437943)
TRIUMPH trial protocol (Lilly):
- TRIUMPH-1 (PMID 37888926, NEJM 2023): 1 / 2 / 4 / 8 / 12 mg subcutaneous weekly, 48 weeks — up to 24.2% weight loss at 12 mg
- TRIUMPH-2 (PMID 37888927): 1–12 mg s.c. weekly, 48 weeks (diabetes + obesity)
- TRIUMPH-3 (PMID 39378016): 1–12 mg s.c. weekly, 89 weeks (long-term safety)
- TRIUMPH-4 (PMID 40609566): 1–12 mg s.c. weekly, 48 weeks (knee osteoarthritis)
Dose escalation: 1 mg → 2 mg → 4 mg → 8 mg → 12 mg over 16–20 weeks (every 4 weeks)
Source: NEJM 2023 + Lilly press releases. Retatrutide is investigational — no approval in the EU or US.
📚 Sources
- Jastreboff AM, Kaplan LM, FrÃas JP, et al. (2023). Triple-Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial. N Engl J Med 389(6):514–526. PubMed
- Elfeki MA, Alkhouri N. (2023). Triple-Hormone-Receptor Agonist Retatrutide for Obesity. N Engl J Med 389(17):1628. (Letter) PubMed
- Jastreboff AM, Kaplan LM, Hartman ML. (2023). Triple-Hormone-Receptor Agonist Retatrutide for Obesity — Reply. N Engl J Med 389(17):1629–1630. PubMed
- Beccuti G, Bioletto F, Parasiliti-Caprino M, et al. (2024). Estimating Cardiovascular Benefits of Tirzepatide in Sleep Apnea and Obesity: Insight from the SURMOUNT-OSA Trials. Curr Obes Rep. PubMed
- Jastreboff AM, le Roux CW, Stefanski A, et al. (2025). Tirzepatide for Obesity Treatment and Diabetes Prevention. N Engl J Med. PubMed
- Giblin K, Kaplan LM, Somers VK, et al. (2026). Retatrutide for the treatment of obesity, obstructive sleep apnea and knee osteoarthritis: Rationale and design of the TRIUMPH registrational clinical trials. Diabetes Obes Metab 28(1):83–93. PubMed
- Bajaj HS, Welch M, Shah P, et al. (2026). Efficacy and safety of retatrutide in people with type 2 diabetes and inadequate glycaemic control (TRANSCEND-T2D-1): a double-blind, randomised, phase 3 trial. Lancet (Online ahead of print). PubMed
- Coskun T, Wu Q, Schloot NC, et al. (2025). Effects of retatrutide on body composition in people with type 2 diabetes: a phase 2 substudy. Lancet Diabetes Endocrinol 13(8):674–684. PubMed
- Retatrutide improves steatohepatitis in an accelerated mouse model of diet-induced steatohepatitis. (2025). Am J Physiol Gastrointest Liver Physiol. (Preclinical — animal model) PubMed
Funding & Regulatory Note: Retatrutide (LY3437943) is an investigational peptide developed by Eli Lilly — currently in Phase 3 trials (TRIUMPH program for obesity/OSA/OA; TRANSCEND-T2D-1 for type 2 diabetes). Not yet approved in any indication. Phase 3 obesity results have not been peer-reviewed or published. Concrete FDA/EMA approval timelines as of 06/2026 have not been officially communicated. The frequently quoted "28.7%" figures come from conference presentations, not from the peer-reviewed publications linked above.