Retatrutide Side Effects: What the Trials Reported

The most common side effects of retatrutide are nausea, diarrhea, constipation and vomiting, and they get more frequent at higher doses. In Lilly's TRIUMPH-1 trial, 42.4% of people on the top 12 mg dose had nausea and 25.3% vomited, compared with 14.8% and 4.8% on placebo. Retatrutide also causes dysesthesia, an unusual skin sensitivity, in about 1 in 8 people on the higher doses. About 11% of people on 12 mg stopped treatment because of side effects.

Retatrutide is still investigational, so there's no approved label yet. This page summarizes what the clinical trials have reported, what to watch for, and why products sold online carry extra risks.

Lilly describes most of these as mild to moderate and says most resolved during treatment. Gastrointestinal side effects were most common while the dose was being raised; doses started at 2 mg and went up every four weeks.

Retatrutide shares the GLP-1 effects of semaglutide and tirzepatide: it slows stomach emptying and reduces appetite. That's where the nausea, fullness, burping, diarrhea and constipation come from. At the top dose, the rates look somewhat higher than in the main semaglutide and tirzepatide trials, though those trials aren't directly comparable.

Repeated vomiting or diarrhea can cause dehydration, which in turn can strain the kidneys. That's the practical risk to watch, more than the discomfort itself.

Dysesthesia means abnormal skin sensations: tingling, burning, sensitivity to touch or clothing. It's not a typical side effect of semaglutide or tirzepatide, and it's one of the signals FDA reviewers are likely to look at closely. In TRIUMPH-1 it affected about 12% of people on 9 mg and 12 mg versus under 1% on placebo, was generally mild to moderate, and usually went away while people stayed on treatment. Researchers haven't settled on why it happens.

Like other incretin drugs, retatrutide raises resting heart rate. In the Phase 2 study, the increase was dose-dependent, peaked around 24 weeks and then eased. In TRIUMPH-3, a trial in people with obesity and established heart disease, retatrutide improved blood pressure, triglycerides and inflammation markers, but didn't significantly reduce major heart events; there were fewer events than expected, so that question isn't settled.

If you think someone has taken too much, see our overdose page and call Poison Control (1-800-222-1222 in the U.S.) or emergency services.

Everything above comes from controlled trials using Lilly's product, slow dose increases and medical monitoring. Vials sold online as retatrutide, "GLP-3" or "reta" for research use have none of that. The dose, purity and sterility are unknown, and FDA says retatrutide can't legally be compounded. The real side-effect risk of those products can't be estimated from the trial data.

Retatrutide's side effects follow the familiar GLP-1 pattern, mostly nausea, diarrhea, constipation and vomiting, but at somewhat higher rates on the top dose, plus a distinctive skin-sensitivity effect. Most cases were mild to moderate in trials, yet about one in nine people on 12 mg stopped treatment. For the efficacy side of the story, see retatrutide weight loss results, and for timing, our approval watch.