If semaglutide introduced single-receptor GLP-1 drugs and tirzepatide added a second receptor, retatrutide is the next step: a triple agonist that targets three receptors at once. It is still investigational, but its phase 3 weight-loss numbers are the largest reported for any drug in this class so far. Here is what it is and how it works.
For context on the drugs that came before it, see tirzepatide's mechanism, how semaglutide works, and tirzepatide vs semaglutide.
What Is Retatrutide?
Retatrutide (development code LY3437943) is an investigational peptide from Eli Lilly, described as a first-in-class triple hormone receptor agonist. It is a single molecule engineered to activate three receptors: GIP, GLP-1, and glucagon. Like the other drugs in this class, it is given as a once-weekly subcutaneous injection.
How Retatrutide Works
Retatrutide combines the incretin effects of the earlier drugs with a third mechanism:
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GLP-1 receptor: glucose-dependent insulin release, reduced glucagon after meals, slower gastric emptying, and appetite suppression in the brain.
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GIP receptor: added insulin support and effects on fat-tissue metabolism, as in tirzepatide.
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Glucagon receptor: the new ingredient. Beyond its role in blood sugar, glucagon-receptor activation increases energy expenditure and promotes fat breakdown in the liver. Adding it is thought to boost the metabolic rate rather than only reducing calorie intake.
The theory is that reducing how much you eat (GLP-1/GIP) while also increasing how much energy you burn (glucagon) produces larger weight loss than incretin action alone.
Single vs Dual vs Triple
| Drug | Receptors | Class |
|---|---|---|
| Semaglutide | GLP-1 | Single agonist |
| Tirzepatide | GIP + GLP-1 | Dual agonist |
| Retatrutide | GIP + GLP-1 + glucagon | Triple agonist |
What the Phase 3 Trials Showed
Retatrutide is being tested in Eli Lilly's phase 3 TRIUMPH program. In TRIUMPH-1 — a randomized, placebo-controlled trial of 2,339 adults with obesity (without diabetes) — average weight loss at 80 weeks was:
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12 mg: about 28.3% of body weight
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9 mg: about 25.9%
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4 mg: about 19.0%
In a subgroup followed to 104 weeks, the 12 mg arm reached about 30.3%. These are the highest averages reported for this drug class to date, though the 30% figure applies to an extension subgroup rather than the whole trial. Additional trials (TRIUMPH-2 and TRIUMPH-3) have also been reported, extending the data in diabetes and other populations.
Side Effects
Retatrutide's side-effect profile looks like a stronger version of the existing drugs, and it is dose-dependent. Gastrointestinal effects were the most common — nausea affected roughly 29% to 42% of participants depending on dose, and vomiting rose with dose. Discontinuations due to adverse events also rose with dose, reaching about 11% at the highest dose. Some participants reported transient tingling sensations (dysesthesia). As an investigational drug, its long-term safety is not yet established.
Current Status
This is the most important point: retatrutide is not FDA approved and is not available by prescription. It is an investigational drug still in clinical trials. It is also not a legitimate research-chemical product — any product sold online claiming to be retatrutide is unregulated and unverified. Approval, if it comes, would follow completion of the phase 3 program and FDA review.
Frequently Asked Questions
What is retatrutide?
An investigational Eli Lilly drug that is a triple agonist, activating the GIP, GLP-1 and glucagon receptors. It is given as a weekly injection and is still in clinical trials.
How is retatrutide different from tirzepatide?
Tirzepatide activates two receptors (GIP and GLP-1). Retatrutide adds a third, the glucagon receptor, which is thought to increase energy expenditure on top of reducing appetite.
How much weight did retatrutide produce in trials?
In the phase 3 TRIUMPH-1 trial, average weight loss at 80 weeks was about 28% at the 12 mg dose, with a 104-week extension subgroup reaching about 30%.
Is retatrutide available or FDA approved?
No. It is investigational, not FDA approved, and not available by prescription. Products sold online claiming to be retatrutide are unregulated.
Disclaimer
This article is for educational and informational purposes only and is not medical advice. Retatrutide is an investigational drug that is not approved by the FDA and not available by prescription. Trial results describe outcomes under controlled study conditions and are not a recommendation. The Peptide Library is an educational resource.
References
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Retatrutide Achieves Up to 30.3% Average Weight Loss in TRIUMPH-1, AJMC, 2026.
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Lilly: Retatrutide Phase 3 Obesity Results, Eli Lilly, 2026.
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Jastreboff AM, et al. Triple–Hormone-Receptor Agonist Retatrutide for Obesity (phase 2). New England Journal of Medicine, 2023.