Overview
Retatrutide is a synthetic 39-amino-acid peptide developed by Eli Lilly that simultaneously activates three receptors: the glucose-dependent insulinotropic polypeptide (GIP) receptor, the glucagon-like peptide-1 (GLP-1) receptor, and the glucagon receptor. This triple agonism represents an expansion beyond tirzepatide's dual mechanism and is designed to further amplify metabolic effects. As of 2026, retatrutide is in Phase III clinical development. Phase II data (NEJM, 2023) reported mean weight loss of 24.2% at the highest dose over 48 weeks — the largest published weight reduction for any single pharmacological agent in a randomized controlled trial.
Mechanism
Beyond the GIP and GLP-1 receptor actions shared with tirzepatide, retatrutide's glucagon receptor agonism adds increased hepatic glucose production suppression, direct thermogenic effects on brown adipose tissue, and enhanced lipolysis. The glucagon receptor component drives additional energy expenditure and fat oxidation, theoretically explaining the incremental weight loss beyond tirzepatide. Balancing glucagon agonism without causing hyperglycemia requires precise receptor selectivity ratios — a key engineering challenge Eli Lilly addresses through tirzepatide's structural modifications.
Research Areas
Side Effects (Preclinical)
- – Nausea and vomiting (most common, especially on dose escalation)
- – Diarrhea
- – Decreased appetite
- – Injection-site reactions
Cautions
- – Investigational compound — Phase III data not yet complete
- – Research-grade material for laboratory research only
- – Not approved for any therapeutic use as of 2026
What the research shows
The Phase II obesity trial (Jastreboff et al., 2023) reported 24.2% mean weight loss at the 12 mg dose over 48 weeks — exceeding tirzepatide’s SURMOUNT-1 results at a comparable timepoint. The study also showed dose-dependent improvements in waist circumference, blood pressure, and lipid profiles.
Retatrutide represents the current frontier of incretin-based research, and its Phase III data (expected 2025–2026) will clarify whether triple agonism offers a clinically meaningful advantage over dual agonism.
References
- 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 — Rosenstock J et al. (2023)
- Triple hormone receptor agonist retatrutide for obesity — a phase 2 trial — Jastreboff AM et al. (2023)