Beyond tirzepatide — the triple agonist
Retatrutide (LY3437943) is Eli Lilly's next-generation obesity drug — a single molecule that simultaneously activates all three key incretin hormone receptors: GLP-1, GIP, and glucagon. While tirzepatide (Mounjaro/Zepbound) combined GLP-1 and GIP to produce superior weight loss over semaglutide, retatrutide adds glucagon receptor agonism to the dual incretin platform. Glucagon is the "counter-regulatory" hormone normally seen as the opposite of insulin — but at the receptor level, glucagon agonism in this context increases energy expenditure, drives hepatic fat reduction, and reduces LDL cholesterol through PCSK9 degradation. This is the rationale for the triple combination.
The Phase 2 NEJM trial (2023, n=338, published simultaneously in NEJM) was the most impressive weight loss trial data ever published at that point: 24.2% mean body weight reduction at 48 weeks in the 12mg dose group versus 2.1% placebo. 72% of participants with prediabetes reverted to normoglycemia. 90%+ with NAFLD achieved liver fat normalisation at the highest dose. A parallel Nature Medicine (2024) substudy showed 82% reduction in liver fat.
December 11, 2025: Eli Lilly announced positive Phase 3 TRIUMPH-4 topline results — 28.7% weight loss at 68 weeks in adults with obesity and knee osteoarthritis, with significant reductions in joint pain. This is the first Phase 3 success for retatrutide. Seven more Phase 3 readouts are expected in 2026. FDA approval is forecast by GlobalData for 2027.
Why glucagon adds value: Glucagon's role extends far beyond blood sugar counter-regulation. Glucagon receptors in the liver drive hepatic fat oxidation and reduce triglycerides. Glucagon increases energy expenditure independently of appetite suppression. Glucagon agonism also degrades PCSK9 — reducing LDL cholesterol by approximately 20%. These are effects neither GLP-1 nor GIP provides. The triple combination is genuinely mechanistically additive, not merely pharmacologically redundant.
Three receptors, one molecule
Mechanism of Action
The Phase 2 data was remarkable across multiple outcomes. At the 12mg dose: 24.2% weight loss at 48 weeks; 100% of participants achieved ≥5% weight loss; 83% achieved ≥15% weight loss; 72% of prediabetics reverted to normoglycemia; and 90%+ with NAFLD normalised liver fat. The meta-analysis of Phase 2 RCTs (878 patients) confirmed mean weight reduction of 14.33% across all doses.
Phase 3 TRIUMPH-4 (December 2025): 28.7% mean weight loss at 68 weeks (12mg dose), with 26.4% at 9mg. Significant joint pain reduction (75.8% improvement in WOMAC pain score). LDL and triglycerides reduced. Systolic blood pressure reduced by 14mmHg at highest dose. Seven additional Phase 3 readouts expected across 2026.
What people report
"The appetite suppression was the most complete I've experienced across any GLP-1 class drug. Not just reduced hunger — a genuine absence of food preoccupation. The weight loss was faster than my experience with semaglutide. GI side effects were real at dose escalation but manageable."
Reported from Phase 2 trial participant accounts. Unlike most entries in this book, community use of retatrutide is essentially non-existent — it is not available outside clinical trials. This is appropriate: it remains an investigational drug and should not be used outside study settings.
"The glucagon component is what makes retatrutide genuinely different. We're not just seeing more of the same appetite suppression — we're seeing energy expenditure increases and PCSK9-mediated LDL reductions that don't come from GLP-1 or GIP alone. If the Phase 3 cardiovascular data comes in positive, this could become the first-line obesity drug."
Endocrinologist perspective on the Phase 2 data. The cardiovascular outcome trial (TRIUMPH-CV) is expected to be the most consequential Phase 3 readout — if retatrutide shows superior cardiovascular benefit to tirzepatide, it would represent a significant advancement over the current standard of care.
What the data shows
Risks & considerations
⚠ Key Warnings
When approved — preparation now
Retatrutide is not yet available outside clinical trials. This section covers what is known about optimising its effects based on Phase 2 data and the class experience from semaglutide and tirzepatide — to be ready when it does receive approval.
Disclaimer: Retatrutide is an investigational drug in Phase 3 trials. It is not approved by any regulatory authority as of April 2026. Do not attempt to source from unregulated markets. Monitor the TRIUMPH programme for Phase 3 results and FDA approval timeline.
Editor's summary
Retatrutide is probably the most consequential drug in late-stage development for metabolic disease. 28.7% weight loss in Phase 3 is beyond what anyone predicted was achievable pharmacologically — surpassing bariatric surgery comparators in many analyses. The glucagon component adds genuinely new mechanisms (hepatic fat oxidation, energy expenditure, PCSK9/LDL reduction) that GLP-1 and GIP agonism cannot provide.
The challenges are real: higher discontinuation rates at high doses, the universal GLP-1 class concerns about muscle loss and GI tolerability, and the need for seven additional Phase 3 trials to characterise the full benefit-risk profile. The cardiovascular outcomes trial will be decisive — if TRIUMPH-CV delivers results comparable to the SELECT trial for semaglutide, retatrutide may become the reference standard for obesity pharmacotherapy.
FDA approval is expected 2027. Until then, this is a drug to watch, not to use. The community should not attempt to source pre-approval retatrutide — any available material cannot be verified and the Phase 3 safety database is still being established.