Retatrutide: the triple GIP/GLP-1/glucagon receptor agonist

Retatrutide (LY3437943) is an experimental peptide agonist in development by Eli Lilly, studied for its activity at three metabolic hormone receptors. Unlike compounds that activate one or two hormonal receptors, retatrutide simultaneously activates the GLP-1, GIP and glucagon receptors — a combination that places it among the most interesting molecules in metabolic receptor research. Important: retatrutide is still an investigational molecule and is not approved by any regulatory agency. The data in this article comes from scientific publications and clinical trials and is not medical advice.

What is retatrutide?

Retatrutide is a single-molecule triple receptor agonist active at the GLP-1 receptor (GLP-1R), the GIP receptor (GIPR) and the glucagon receptor (GCGR). The simultaneous engagement of all three receptor systems is what sets it apart from earlier generations of incretin therapies. GLP-1 and GIP are hormones tied to glucose regulation and energy balance, while glucagon plays an important role in mobilising energy substrates. The concept: combining these three signals in one molecule may produce a broader metabolic effect.

Why do the three receptors matter?

• GLP-1 — involved in appetite regulation and glucose metabolism; GLP-1 receptor agonists are already an established therapeutic class;
• GIP — an incretin hormone involved in postprandial insulin regulation; the GLP-1/GIP combination proved its clinical potential through tirzepatide;
• Glucagon — stimulates mobilisation of energy reserves and increased energy expenditure; this is retatrutide's most interesting feature — the molecule aims to influence both energy intake and energy metabolism at once.

What do the clinical studies show?

The first major human data came from a 48-week, placebo-controlled Phase 2 trial in 338 adults, published in NEJM in 2023, which reported dose-dependent changes in body weight and metabolic markers. Full results are in the cited publication.

Retatrutide and type 2 diabetes

A Phase 2 trial in 281 people with type 2 diabetes reported dose-dependent changes in HbA1c and body weight.

What do the newer Phase 3 trials show?

In 2026 Lilly reported topline results from several Phase 3 trials in the TRIUMPH programme, in populations with obesity, type 2 diabetes and knee osteoarthritis. These are company announcements; full peer-reviewed publications are pending.

What adverse reactions were observed?

As with other incretin-system therapies, the most commonly reported adverse reactions are gastrointestinal: nausea, diarrhoea, vomiting, constipation — mostly mild to moderate and more frequent in higher-dose trial arms. A dose-dependent increase in heart rate was also observed, peaking around week 24 and declining thereafter. Efficacy results should never be read apart from the safety profile.

What we still do not know

• long-term safety;
• durability of the observed metabolic changes;
• effects after stopping therapy;
• long-term cardiovascular and metabolic outcomes;
• safety in specific populations;
• comparison against established therapies.

As of September 2026, Lilly states that retatrutide is not approved by the FDA or any other regulatory agency and is being studied in a Phase 3 programme.

Retatrutide versus GLP-1 and dual agonists

The evolution can be sketched as: GLP-1 → GLP-1 + GIP → GLP-1 + GIP + glucagon. Semaglutide is a GLP-1 receptor agonist; tirzepatide activates GLP-1 and GIP; retatrutide adds the glucagon receptor. That does not automatically make one molecule “better” in every situation — each combination has its own pharmacology, efficacy and safety profile, and head-to-head trials will determine retatrutide's exact place among modern metabolic therapies.

Conclusion

Retatrutide is one of the most interesting examples of the next generation of peptide metabolic therapies — simultaneous activation of the GLP-1, GIP and glucagon receptors, combining incretin effects with glucagon's metabolic actions. Phase 2 data and 2026 Phase 3 topline results exist for defined clinical populations. Research-grade material is a laboratory reagent and is not the investigational medicine used in those trials.

1. 1.Jastreboff AM, Kaplan LM, Frías JP, et al. Triple-Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial. N Engl J Med. 2023;389(6):514–526.
2. 2.Rosenstock J, Frias J, Jastreboff AM, 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;402(10401):529–544.
3. 3.Abdrabou Abouelmagd A, et al. Efficacy and safety of retatrutide, a novel GLP-1, GIP, and glucagon receptor agonist: a systematic review and meta-analysis. Proc (Bayl Univ Med Cent). 2025.
4. 4.Panou T, Gouveri E, et al. Retatrutide in type 2 diabetes mellitus and obesity: an overview. Expert Rev Clin Pharmacol. 2026.
5. 5.Abdul-Rahman T, Roy P, et al. The power of three: Retatrutide's role in modern obesity and diabetes therapy. Eur J Pharmacol. 2024.
6. 6.Eli Lilly and Company — What to know about retatrutide (clinical development, 2026).
7. 7.Eli Lilly and Company — TRIUMPH-1 and TRIUMPH-4 topline announcements (2026).
8. 8.Eli Lilly and Company — TRIUMPH-2 and TRIUMPH-3 topline announcement (July 2026).