What Is Retatrutide? The Triple-Hormone Weight-Loss Drug Explained

Written by Olha Orlyk. Updated October 22, 2026.

Retatrutide is an experimental weight-loss drug from Eli Lilly, given as a once-weekly injection, that activates three hormone receptors at once: GIP, GLP-1 and glucagon. That's why it's nicknamed a "triple agonist" and sometimes, informally, "GLP-3." In Phase 3 trials, people with obesity lost an average of up to 28.3% of their body weight over 80 weeks. It is not approved by FDA yet; Lilly plans to file by the end of the first quarter of 2027.

Here's what retatrutide does in the body, what the trials have shown, the side effects, and what to make of the versions being sold online.

Retatrutide copies the action of three natural hormones. Each one does something different, and together they push weight down harder than any single- or dual-hormone drug so far.

In practice, people on retatrutide feel full sooner and think about food less, as with Ozempic or Zepbound. The glucagon component is the new ingredient. It's believed to raise the number of calories the body burns and to clear fat from the liver, which may explain both the extra weight loss and the dramatic liver-fat drops seen in early studies. For a side-by-side with the drugs people already know, see retatrutide vs tirzepatide.

Lilly is studying retatrutide for several conditions and plans to seek approval first for obesity, knee osteoarthritis pain and obstructive sleep apnea. It's also being tested in type 2 diabetes, heart disease and kidney disease. If it's approved, each use will be approved separately, based on its own trial results.

The full numbers, including how many people lost 30% or more, are in our retatrutide weight loss results breakdown.

The most common side effects are the familiar ones from this drug family: nausea, diarrhea, constipation and vomiting, mostly while the dose is increased. They were more frequent at higher doses; in TRIUMPH-1, 42.4% of people on 12 mg had nausea. Retatrutide also causes dysesthesia, a tingling or burning skin sensitivity, in about 12% of people on the higher doses. Heart rate rises modestly. See the full side-effects guide.

Not legally outside a clinical trial. Retatrutide hasn't been approved, and FDA says it can't be compounded. Products sold online as retatrutide, "reta" or "GLP-3" for "research use only" are not the drug used in Lilly's trials; their dose, purity and sterility are unknown. For the regulatory timeline, see our approval watch.

You may also see retatrutide sold online under the name "GLP-3 RT." That is not a new molecule or an approved product; our guide to what GLP-3 RT actually is explains where the name came from and why those vials are not the trial drug.

The question that follows what retatrutide is tends to be how quickly it works. Our week-by-week timeline, how fast does retatrutide work, pieces together the phase 2 curves with the 80-week and 104-week TRIUMPH-1 results and compares them with semaglutide and tirzepatide at the same timepoints.

Retatrutide is Lilly's next-generation obesity drug: a weekly injection that adds glucagon to the GLP-1 and GIP effects of tirzepatide. It has produced the largest weight loss of any obesity drug in trials, with side effects similar to the class but somewhat more frequent at the top dose. It isn't approved yet; a filing is expected by early 2027.