Retatrutide: The Triple-Agonist Trial Data
What the peer-reviewed phase 2 trials actually show — and where the viral “phase 3” dose tables really stand.
How It Works
Retatrutide (LY3437943) is a single synthetic peptide that agonizes the GIP, GLP-1 and glucagon receptors at once — the defining "triple agonist" mechanism that separates it from GLP-1 (semaglutide) and GLP-1/GIP (tirzepatide) drugs.
The GLP-1 and GIP arms slow gastric emptying, enhance glucose-dependent insulin secretion and reduce appetite — the same axis that drives weight loss and HbA1c reductions in the incretin drug class.
Glucagon-receptor agonism is thought to raise energy expenditure and act directly on hepatic fat, and may contribute to retatrutide’s unusually large liver-fat reductions in the phase 2a MASLD substudy — though the substudy authors offer it as one possible explanation, not an established one.
Across the phase 2 trials, the largest reductions in body weight, HbA1c and liver fat were seen at the higher doses tested, up to 12 mg — which is why phase 3 TRIUMPH carries the top of that range forward: all four TRIUMPH trials test 9 mg and 12 mg weekly, with a 4 mg maintenance dose included only in TRIUMPH-1 and TRIUMPH-2.
What the Data Shows
Key Takeaways
- Retatrutide (LY3437943, Eli Lilly) is an investigational once-weekly injectable triple agonist of the GIP, GLP-1 and glucagon receptors.
- In the 48-week phase 2 obesity trial (338 adults, NEJM 2023), the 12 mg dose produced a mean −24.2% body-weight change vs −2.1% for placebo; 83% of the 12 mg group lost ≥15% of body weight.
- In the phase 2 type-2-diabetes trial (281 adults, Lancet 2023), the 12 mg dose lowered HbA1c by 2.02% at 24 weeks alongside robust bodyweight reductions.
- In a phase 2a MASLD substudy (98 adults, Nature Medicine 2024), the 12 mg dose cut liver fat 82.4% at 24 weeks and 86% of that group reached normal liver fat (<5%).
- Gastrointestinal effects (nausea, diarrhea, vomiting) were the most common adverse events — dose-related, mostly mild-to-moderate, and partly reduced by a lower 2 mg starting dose.
- The phase 3 registrational program, TRIUMPH — covering obesity, obstructive sleep apnea and knee osteoarthritis — has begun reporting (TRIUMPH-1 in NEJM and TRIUMPH-2 in The Lancet, September 2026); its design was published in Diabetes, Obesity and Metabolism (Jan 2026).
- Phase 3 headline numbers differ by estimand: the peer-reviewed TRIUMPH-1 paper (NEJM 2026) reports −25.0% on 12 mg at 80 weeks (intention-to-treat), while Lilly's topline led with a 28.3% efficacy-estimand figure — check which one a source is quoting.
- Retatrutide is NOT FDA-approved. It is available only through clinical trials, not as a prescription or an approved product.
- Long-term safety, durability of weight loss, and the degree of weight regain after stopping are not established from 48-week phase 2 data.
- Dose-dependent heart-rate increases were observed (peaking around 24 weeks, then declining); cardiovascular outcome data are not yet available.
- Superiority over tirzepatide or semaglutide has not been proven in a head-to-head trial — cross-trial percentage comparisons are unreliable.
Frequently Asked Questions
What were retatrutide’s phase 2 weight-loss results?
In the 48-week phase 2 obesity trial (N=338, NEJM 2023), least-squares mean body-weight change was −8.7% (1 mg), −17.1% (4 mg), −22.8% (8 mg) and −24.2% (12 mg) versus −2.1% for placebo. In the 12 mg group, 100% lost ≥5%, 93% lost ≥10% and 83% lost ≥15% of body weight.
Are the retatrutide TRIUMPH phase 3 results published?
Yes. TRIUMPH-1 (adults with obesity without diabetes) was published in the New England Journal of Medicine and TRIUMPH-2 (adults with obesity and type 2 diabetes) in The Lancet, both on 29 September 2026. In TRIUMPH-1, mean body-weight change at 80 weeks was −25.0% on the 12 mg dose under the intention-to-treat (treatment-regimen) estimand. The 28.3% figure in Lilly's topline release (May 21, 2026) is the efficacy estimand from the same trial, not a separate result. Sources: TRIUMPH-1, NEJM (PMID 42814954); TRIUMPH-2, Lancet (PMID 42810372). Full breakdown: retatrutide TRIUMPH-1 results.
How does retatrutide compare to tirzepatide and semaglutide?
Retatrutide adds a third mechanism — glucagon-receptor agonism — on top of the GLP-1 (semaglutide) and GLP-1/GIP (tirzepatide) actions, which may help explain its large phase 2 weight-loss and liver-fat numbers. But those figures come from separate phase 2 trials, so direct percentage comparisons across drugs are not reliable without a head-to-head study.
Is retatrutide FDA-approved or available by prescription?
No. Retatrutide is investigational and is not an approved medication; the only sanctioned access is through Eli Lilly’s clinical trials. It cannot be prescribed as a weight-loss drug, and nothing on this page is a recommendation to obtain it.
What are retatrutide’s side effects?
The most common adverse events in the phase 2 trials were gastrointestinal — nausea, diarrhea and vomiting — which were dose-related and mostly mild to moderate. A lower 2 mg starting dose reduced them. Dose-dependent increases in heart rate were also seen, peaking around 24 weeks before declining. This answer covers the phase 2 trials only; the TRIUMPH-1 and TRIUMPH-2 phase 3 papers report additional safety findings — see those papers for the full adverse-event profile.
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Retatrutide is an investigational compound and is not approved by the FDA. Nothing here is a recommendation to obtain or use it. Consult a licensed clinician about any medical decision.
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