Retatrutide

Retatrutide (LY3437943) is Eli Lilly's investigational triple agonist peptide — the first compound to reach late-stage clinical trials that simultaneously activates the GLP-1, GIP, and glucagon receptors in a single molecule. Where semaglutide hits one incretin pathway and tirzepatide hits two, retatrutide adds glucagon receptor activation to the mix, and that third mechanism is the genuinely novel piece. It changes the metabolic equation from "eat less" to "eat less and burn more."

The glucagon receptor agonism is the defining differentiator. GLP-1 and GIP both lower calorie intake — slower gastric emptying, hypothalamic appetite suppression, food-reward dampening. Useful, but ultimately one side of the energy balance equation. Glucagon receptor activation works on the other side: it raises basal metabolic rate by an estimated 3–5%, stimulates hepatic lipolysis (the liver oxidizing its own fat stores), and increases sympathetic nervous system tone. The net effect is that retatrutide users are pulling both levers simultaneously — eating less while burning more at rest. This is the mechanistic explanation for why the TRIUMPH-1 Phase 2 trial produced the largest mean weight loss ever observed in this drug class: 24.2% at the 12 mg dose over 48 weeks, exceeding tirzepatide's SURMOUNT-1 result of 22.5% over 72 weeks.

The trial data is genuinely impressive. In TRIUMPH-1 (Jastreboff et al., NEJM 2023), every single participant on the 8 mg and 12 mg arms achieved at least 5% weight loss — a 100% response rate that no prior weight-loss drug has produced. 83% of the 12 mg arm achieved 20% or greater loss. A hepatic-fat substudy showed an 82.4% relative reduction in liver fat at 24 weeks, with potential implications for the millions of patients with NAFLD/MASLD. The TRIUMPH-2 Phase 2 trial in type 2 diabetes showed A1c reductions of 2.16% on the 12 mg dose vs 0.01% on placebo — matching or exceeding tirzepatide's diabetes data. Phase 3 trials (TRIUMPH-3 for cardiovascular outcomes, TRIUMPH-4 for obesity with knee osteoarthritis, TRIUMPH-OSA for sleep apnea) are enrolling and expected to read out in the 2025–2026 window, with FDA approval anticipated in late 2026 or 2027.

The status of retatrutide outside of formal clinical trials is genuinely complicated. It is not FDA-approved for any indication, which means compounding pharmacies cannot legally manufacture it under either 503A (patient-specific) or 503B (outsourcing facility) frameworks. Anything sold as "compounded retatrutide" is operating outside the legal compounding framework. The research-chemical market has filled this gap — and the supply chain quality varies dramatically between vendors. Unlike semaglutide and tirzepatide, where compounded versions can be verified against the well-characterized branded reference product, there is no FDA-approved retatrutide reference standard to compare against. Independent third-party verification of identity, purity, and dosage of any non-Lilly-manufactured retatrutide is essentially impossible to confirm.