Retatrutide

Retatrutide (Lilly’s lab code LY3437943) is an experimental injectable drug being studied for obesity and type 2 diabetes. It has shown some of the biggest weight-loss numbers seen in any trial so far. But as of June 2026 it is not approved anywhere, and the big Phase 3 results that matter most have not yet been checked over by independent scientists (peer review).

What it is

Retatrutide is a once-weekly shot given subcutaneously (under the skin). What makes it unusual is that it switches on three different gut and metabolic hormone receptors at the same time: GLP-1, GIP, and glucagon. (A receptor is like a lock on a cell that a hormone fits into to send a signal.) The drugs already on the market hit just one of these (semaglutide) or two of them (tirzepatide). The idea behind adding the glucagon receptor is that it may help the body burn more energy, on top of curbing appetite. It is made by Eli Lilly and is still an experimental compound.

The claims

Online sellers and forums hype retatrutide as the most powerful weight-loss drug yet, throwing around claims like 25 percent or more body-weight loss, a “cleaning out” of liver fat, and even diabetes “reversal.” Some marketing pitches it as the next step up from tirzepatide that you can buy right now. Both sides of that pitch — how well it works and whether you can actually get it — deserve a close look.

What the evidence actually shows

The human data are real, and when it comes to weight loss, they are genuinely strong. In a published 48-week Phase 2 randomized trial of 338 adults with obesity (Jastreboff et al., NEJM, 2023), the 12 mg dose produced about 24 percent mean weight loss (−24.2%), compared with roughly 2 percent (−2.1%) for people taking a placebo (a dummy treatment with no active drug). A separate Phase 2 trial in 281 people with type 2 diabetes (Rosenstock et al., The Lancet, 2023) showed meaningful drops in both HbA1c (a blood-sugar marker that reflects average glucose over a few months) and weight. And a 98-person Phase 2a substudy found that liver fat fell more the higher the dose — by roughly 82 percent at 24 weeks (and about 86 percent by 48 weeks) at the 12 mg dose in people with fatty liver disease.

In May 2026 Lilly announced its first headline Phase 3 results (TRIUMPH-1, roughly 2,300 participants, 80 weeks): about 28 percent average weight loss on 12 mg versus about 2 percent on placebo. A longer extension of the study, where neither patients nor doctors knew who got what (blinded), reached about 30 percent at 104 weeks in a subset of people. An earlier Phase 3 readout in people who had both obesity and knee osteoarthritis (TRIUMPH-4, December 2025) reported similar weight loss — about 29 percent, an average of 71.2 lb at the top dose — along with less knee pain.

Now the important catches. The Phase 3 results so far exist only as company press releases, not as peer-reviewed publications that outside experts have vetted. Several of the TRIUMPH trials had not fully reported their findings as of mid-2026, and there is no long-term safety data and no data yet on heart-related outcomes. In short: this is a promising story, but the evidence is not finished.

Legal and regulatory status

Retatrutide is not approved by the FDA, the EMA (Europe’s drug regulator), or any other major regulator as of June 2026. It is still investigational, which means it cannot legally be prescribed or sold for people to use outside of a clinical trial. Lilly has said it expects to file an application in the U.S. around late 2026, which would put any possible approval no sooner than 2027. Anything sold online as “retatrutide” right now is unapproved and unregulated — there is no guarantee of what is actually in it, how pure it is, or how strong the dose is.

For athletes: retatrutide is not specifically named on WADA’s 2026 Prohibited List, and the GLP-1 class as a whole is not banned. Semaglutide and tirzepatide sit on WADA’s 2026 Monitoring Program — meaning they are being watched for possible misuse — but they are not prohibited. That said, retatrutide is an unapproved experimental substance, and WADA has a separate rule (category S0) that bans any substance not approved for human use. So a competing athlete should treat its status as unsettled and check with their anti-doping authority before using it.

Safety

In the trials, the most common side effects were stomach- and gut-related (gastrointestinal) — nausea, vomiting, diarrhea, and constipation. These were mostly mild to moderate and tended to cluster early on, while the dose was being slowly raised. There was also a small bump in heart rate at higher doses. Notably, liver fat got better rather than worse, and there was no signal of liver damage (hepatotoxicity) through 48 weeks in the studies above. Even so, the long-term safety picture is still unknown — and none of this reassurance applies to gray-market product bought outside the system, which carries extra risks that no one has measured.

Bottom line

Retatrutide has truly impressive trial results and may turn out to be an important obesity drug. But it is still experimental, its Phase 3 data have not yet been peer-reviewed, and it is not legally available. Anything for sale today is unapproved. Nothing here is medical advice.

Evidence grade: 7/10 · Moderate.

Sources

• Jastreboff AM, et al. Triple–Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial. NEJM. 2023.
• Rosenstock J, et al. Retatrutide, a GIP, GLP-1 and glucagon receptor agonist, for people with type 2 diabetes: a phase 2 trial. The Lancet. 2023.
• Sanyal AJ, et al. Triple hormone receptor agonist retatrutide for metabolic dysfunction-associated steatotic liver disease: a randomized phase 2a trial. Nature Medicine. 2024.
• Lilly’s triple agonist, retatrutide, delivered weight loss of up to an average of 71.2 lbs in first successful Phase 3 trial (TRIUMPH-4). Eli Lilly news release. December 2025.
• Lilly’s triple agonist, retatrutide, delivered powerful weight loss in pivotal Phase 3 obesity trial (TRIUMPH-1). Eli Lilly news release. May 2026.
• Retatrutide Achieves Up to 30.3% Average Weight Loss in Phase 3 TRIUMPH-1 Trial. AJMC (reporting Lilly topline, May 2026).
• What to know about retatrutide. Eli Lilly.
• WADA’s 2026 Prohibited List now in force. World Anti-Doping Agency.