Retatrutide is the first triple-incretin agonist in late-stage trials. What the Phase 2 data showed, what Phase 3 is testing, and how it compares to tirzepatide on the metrics that matter.
Quick Summary - Retatrutide (LY-3437943) is Eli Lilly's investigational triple-receptor agonist — it activates GLP-1, GIP, AND glucagon receptors in a single molecule. - The "GLP-3" framing in community discussion refers to this third receptor target. Retatrutide is the first compound in late-stage trials with this profile. - In the Phase 2 obesity trial, the highest dose (12 mg weekly) produced ~24% mean body weight reduction at 48 weeks — the largest weight-loss signal published for any pharmacologic compound to date. - Phase 3 trials (TRIUMPH program) are reading out across obesity, type 2 diabetes, and obstructive sleep apnea. - Not FDA-approved as of April 2026. Currently available only via research-chemical channels with associated quality concerns. - This is informational reporting only. Not medical advice. Always discuss any GLP-class compound with your prescribing physician.
The current FDA-approved GLP class works on two pathways:
Retatrutide adds a third receptor: glucagon. Glucagon is best known as the hormone that raises blood sugar (the opposite of insulin), but it also has direct effects on energy expenditure, hepatic fat metabolism, and lipid metabolism. The bet behind retatrutide is that adding glucagon-receptor activation on top of GLP-1 + GIP creates a metabolic effect that's larger than the sum of the parts.
The published Phase 2 data supports that bet. The mean weight reduction at 48 weeks for the 12 mg dose group was approximately 24% — meaningfully above the ~22.5% seen in the comparable tirzepatide trial (SURMOUNT-1) at maintenance dose, and substantially above the ~15% seen with semaglutide at maintenance dose in the STEP trials.
Lilly's Phase 3 program for retatrutide is one of the most ambitious incretin trial programs ever run. Key trials in the TRIUMPH series:
Lilly has indicated they plan to file for FDA approval after Phase 3 data reads out, with potential regulatory action in 2026 or 2027 depending on timing.
Three points stand out from the published data:
1. Ongoing weight loss curve, not a plateau. In the Phase 2 trial, the weight loss curve at 48 weeks did not appear to flatten in the highest-dose groups. Many existing GLP-class compounds reach a plateau by week 36–48. The retatrutide curve suggested room for further loss with continued treatment — though this needs Phase 3 confirmation.
2. Heart rate elevation. Mean heart rate rose ~4–6 beats per minute in the higher-dose groups during Phase 2. This is something to monitor, particularly for users with pre-existing cardiovascular concerns. Phase 3 cardiovascular outcomes data (TRIUMPH-3) will clarify whether this is a meaningful long-term concern.
3. Lipid and hepatic biomarker improvements. ApoB, triglycerides, and ALT all improved meaningfully in Phase 2 — consistent with the metabolic and hepatic effects of glucagon-receptor activation. The hepatic fat reduction signal in particular is notable.
Anyone running retatrutide (currently research-only, not FDA-approved) should track at minimum:
Pair these with weight tracking, body composition (DEXA if available), and recovery metrics if you have a wearable. Retatrutide's metabolic signature is broad enough that you'll see movement across most of these markers.
The compound may eventually become a first-line option for clinically severe obesity. Until that happens through proper regulatory channels, the realistic positioning is: keep an eye on the trial data, work with a physician, and don't trust gray-market quality.
If you're on any GLP-class compound — semaglutide, tirzepatide, or future retatrutide — MyProtocolStack lets you log doses with the actual peptide library, upload lab PDFs from Quest, LabCorp, BostonHeart, or any major US lab, and watch StackAI read the trends in context of what you're running. Retatrutide is in our compound library and our reconstitution calculator handles the standard 12 mg vial sizes.
[Try the calculator](https://myprotocolstack.com/calculators/retatrutide) · [Set up tracking](https://myprotocolstack.com/auth/login)
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Sources & references: Eli Lilly Phase 2 publications (Jastreboff et al., NEJM, 2023). Lilly press releases for TRIUMPH program updates. Compound profile data from MyProtocolStack peptide library. This article is for informational purposes only and does not constitute medical advice. Discuss any pharmacologic protocol with your physician.
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