Retatrutide (LY3437943) is an investigational, once-weekly, lipidated 39-residue peptide that activates the glucose-dependent insulinotropic polypeptide, glucagon-like peptide-1, and glucagon receptors. Its molecular design combines incretin activity with glucagon receptor engagement while maintaining glycaemic efficacy. This critical narrative review examines molecular engineering, pharmacokinetics, dose escalation, pharmaceutical development, and clinical evidence available through 11 September 2026. Five phase 3 trials have reported positive primary outcomes, but only TRANSCEND-T2D-1 has a full peer-reviewed report; the obesity trials are currently available as sponsor topline disclosures. In those disclosures, efficacy-estimand mean weight reductions of 28.3% at 80 weeks (TRIUMPH-1) and 28.7% at 68 weeks (TRIUMPH-4) were reported, with a TRIUMPH-1 treatment-regimen estimate of 25.0%; these values remain provisional pending full published trial reports. The peer-reviewed TRANSCEND-T2D-1 report supports glycaemic efficacy in early type 2 diabetes. Interpretation requires attention to estimands, population differences, treatment discontinuation and selection into trial extensions. Gastrointestinal adverse events, dysaesthesia, heart-rate changes and the functional consequences of lean-mass loss require further evaluation. Pharmaceutical priorities include scalable synthesis, impurity and potency control, formulation stability, immunogenicity and reliable dose delivery. Retatrutide provides a clinically advanced example of unimolecular poly-agonism, but comparative effectiveness, long-term outcomes and regulatory review remain essential to defining its therapeutic role.
