Retatrutide is an investigational once-weekly injection from Eli Lilly that activates three hormone receptors (GIP, GLP-1 and glucagon). It is not approved by the FDA or any regulator for any use, and it cannot legally be prescribed. In clinical trials it has produced large average weight loss, with digestive side effects as the most common problem. Lilly has said it plans to submit it to the FDA in the first quarter of 2027 (pharmaphorum). Any product sold as “retatrutide” today is not an approved medicine.
What retatrutide is
Retatrutide (code name LY3437943) is an agonist of the GIP, GLP-1 and glucagon receptors (phase 2 trial abstract, NEJM 2023). Approved medicines act on fewer targets: semaglutide (Ozempic, Wegovy) on the GLP-1 receptor (Wegovy label), and tirzepatide (Mounjaro, Zepbound) on GIP and GLP-1 (Mounjaro label, section 12.1). Retatrutide adds the glucagon receptor. Researchers hope this will add effects on energy use, but how much each receptor contributes in people is still under study.
What the trials report
Phase 2 (peer reviewed). In a 48-week trial of 338 adults with obesity or overweight, average weight change at 48 weeks was -8.7% with 1 mg, -17.1% with 4 mg, -22.8% with 8 mg and -24.2% with 12 mg, against -2.1% with placebo. The most common adverse events were gastrointestinal, dose-related and mostly mild to moderate, and a lower starting dose helped. Heart rate rose in a dose-dependent way, peaking at 24 weeks (NEJM 2023, PubMed).
Phase 3 (peer reviewed). Two phase 3 trials were published on September 29, 2026. In TRIUMPH-1, 2,339 adults with obesity and without diabetes were treated for 80 weeks: average weight change was -17.6% with 4 mg, -23.7% with 9 mg and -25.0% with 12 mg, against -3.9% with placebo (NEJM 2026, PubMed). In TRIUMPH-2, 1,152 adults with type 2 diabetes and a BMI of 27 or more were treated for 80 weeks: average weight change was -11.9%, -16.8% and -18.8% on the same doses, against -5.1% with placebo (Lancet 2026, PubMed). Lilly has announced results from other phase 3 trials in press releases; those are not peer reviewed yet, so we don’t repeat them here.
How this compares to approved drugs
For context only: the Wegovy label reports an average of -14.9% at 68 weeks versus -2.4% with placebo (Wegovy label, Table 8), and the Zepbound label reports -20.9% at 72 weeks with 15 mg versus -3.1% with placebo (Zepbound label, section 14). Different trials enrolled different people for different lengths of time, so these figures are not a head-to-head comparison. See our guides to semaglutide and tirzepatide.
Beyond weight: other measures
Trials of drugs in this class also track blood sugar and other health markers. In TRIUMPH-1, retatrutide reduced knee pain in the 574 participants with knee osteoarthritis and reduced breathing interruptions during sleep (the apnea-hypopnea index) in the 243 participants with obstructive sleep apnea, compared with placebo (NEJM 2026). TRIUMPH-2 reported improved blood sugar control in people with type 2 diabetes (Lancet 2026). We haven’t verified published data on liver fat or blood pressure for retatrutide, so we don’t make claims about them.
Side effects reported so far
In the phase 2 trial, the most common adverse events were gastrointestinal, mostly mild to moderate and linked to dose, and heart rate increased with dose before declining (NEJM 2023). In TRIUMPH-2, diarrhea was reported by 27% to 34% of people on retatrutide versus 13% on placebo, and nausea by 14% to 28% versus 8%. Low blood pressure and dysesthesia (an abnormal skin sensation) were also more frequent than with placebo, and 4% to 12% stopped treatment because of adverse events or death, versus 5% on placebo (Lancet 2026). Digestive effects lead the list, as with other GLP-1 drugs: see GLP-1 side effects: what to expect and how to manage them. The long-term safety profile of retatrutide is not established.
Can you get retatrutide?
Not as an approved medicine. The only legitimate way to receive it is by joining a registered clinical trial, which you can search for at ClinicalTrials.gov. Eligibility is set by each trial and your doctor can help you read the criteria. Products sold online as retatrutide are not approved or verified, and we recommend not buying them: you can’t know what they contain or at what dose.
What to do while you wait
If your goal is weight management, ask a prescriber about approved options such as Wegovy, Zepbound or Saxenda (see our GLP-1 weight loss medication guide). If you take part in a trial, keep your own record alongside the study visits. Mingo is a GLP-1 tracker for iPhone and Android that logs shots and doses, food and protein, side effects and weight in one place; it doesn’t replace your doctor or the trial team. Get the app.
Key takeaways
- Retatrutide is investigational: not approved by any regulator and not legally prescribable.
- It activates GIP, GLP-1 and glucagon receptors. Average weight change with 12 mg was -24.2% at 48 weeks in phase 2 and -25.0% at 80 weeks in the phase 3 TRIUMPH-1 trial.
- Digestive side effects were the most common; long-term safety is unknown.
- Retatrutide is not approved by the FDA. Whether and when it could be is not something we can predict.
Frequently asked questions
What is retatrutide used for?
It is being studied for obesity and type 2 diabetes. It has no approved use.
How does retatrutide help with weight loss?
It activates the GIP, GLP-1 and glucagon receptors. Trials show large average weight loss; how each receptor contributes in people is still being studied.
What are the side effects of retatrutide?
In phase 2 and phase 3 trials, gastrointestinal events such as diarrhea and nausea were the most common, and in phase 2 heart rate rose with dose. Long-term safety is not known.
How much does retatrutide cost?
There is no price because it is not approved. Trial participants receive study medication under the trial’s terms.
Is retatrutide FDA approved?
No. It is investigational. Lilly has said it plans to submit it to the FDA in the first quarter of 2027, and the FDA has not decided on it.
