Retatrutide (development code LY3437943) is an investigational drug from Eli Lilly. It is a single synthetic peptide that activates three receptors at the same time: the GLP-1 receptor, the GIP receptor, and the glucagon receptor. That third target is what sets it apart from semaglutide (GLP-1 only) and tirzepatide (GLP-1 plus GIP). Engaging the glucagon receptor is thought to raise energy expenditure on top of the appetite and glucose effects of the other two.
It is given as a once-weekly subcutaneous injection. As of mid-2026 it is not approved by the FDA, EMA, or any other regulator. It exists only inside clinical trials.
How it works
A structural study published in 2024 describes retatrutide as a unimolecular triple agonist — one peptide that binds and activates GLP-1R, GIPR, and GCGR. Its potency is not balanced across the three. The same work reports it is more potent at the GIP receptor than the body's own GIP, and somewhat less potent than the native hormones at the glucagon and GLP-1 receptors. The combined effect on glucose control, appetite, and energy use is the working hypothesis behind its development, and it is still being tested.
What the trials show
The headline data come from a Lilly-sponsored Phase 2 trial in adults with obesity, published in the *New England Journal of Medicine* in 2023. Participants were randomized across several dose groups and placebo. Mean change in body weight at 48 weeks:
| Group | Weight change at 48 weeks |
|---|---|
| Placebo | -2.1% |
| Retatrutide 1 mg | -8.7% |
| Retatrutide 4 mg | -17.1% |
| Retatrutide 8 mg | -22.8% |
| Retatrutide 12 mg | -24.2% |
At the 12-mg dose, every participant lost at least 5% of body weight and 83% lost 15% or more. A separate Phase 2 trial in people with type 2 diabetes (The Lancet, 2023) reported reductions in HbA1c and body weight versus placebo and versus dulaglutide. A Phase 2a study in metabolic dysfunction-associated steatotic liver disease (Nature Medicine, 2024) reported large reductions in liver fat.
The most common side effects across trials were gastrointestinal — nausea, diarrhea, vomiting, constipation. They were dose-related and mostly mild to moderate, and were reduced by starting at a lower dose and titrating up.
Results have since moved into Phase 3. Lilly's TRIUMPH program reported its first positive topline readouts in late 2025 and 2026, with weight reductions in the high-20% range at 68 to 80 weeks. Those are company topline announcements; the full peer-reviewed Phase 3 papers are the figures to wait for.
Regulatory and anti-doping status
Retatrutide is investigational. It has no marketing authorization anywhere, which means any material sold outside a clinical trial is not an approved medicine and carries no regulatory guarantee of identity, purity, or sterility. Vendors typically label it for research use only, not for human consumption.
For athletes: GLP-1 receptor agonists are named on the WADA Prohibited List under section S2 (peptide hormones), and because retatrutide is not approved by any health authority it also falls under S0 (non-approved substances). Treat it as banned in tested sport.
The quality angle
Because approved retatrutide does not exist on the consumer market, anything labeled as such comes from research-chemical suppliers. There is no manufacturer COA to point to the way there is for an approved drug. If you are evaluating a vendor's product, the things that actually matter are third-party analytical data: a recent certificate of analysis tied to the specific batch, identity confirmation (typically mass spectrometry), and purity by HPLC. peptideone aggregates the public COAs and independent vendor ratings we can find so those claims can be checked rather than taken on trust.
Nothing here is medical or dosing advice. peptideone does not sell peptides and runs no tests of its own; the figures above are attributed to the cited primary sources.