Retatrutide is an experimental weekly injection from Eli Lilly that acts on three hormone receptors: GIP, GLP-1 and glucagon. Its trial weight-loss results have drawn huge interest. But a drug isn't judged safe until FDA has reviewed the full data and approved a label, and retatrutide hasn't reached that point [1]. This page covers what the published trials report about side effects, what isn't known yet, and why FDA warns against the retatrutide sold online.
FDA hasn't approved retatrutide, and says it can't be compounded
FDA's position is plain. On its page about unapproved GLP-1 drugs, the agency says: "Retatrutide and cagrilintide cannot be used in compounding under federal law. Additionally, these are not components of FDA-approved drugs and have not been found safe and effective for any condition" [1]. Retatrutide isn't on FDA's list of new drugs approved this year either [2].
That means there is no prescribing information yet: no official list of side effects, warnings, or people who shouldn't take it. Everything known about its safety comes from clinical trials, all funded by Lilly. What that means for getting it is covered on our retatrutide legal status page.
The phase 2 trial: stomach side effects that rose with dose
The first large trial, published in 2023, gave 338 adults with obesity retatrutide or placebo for 48 weeks. The most common side effects were gastrointestinal, such as nausea, diarrhea and vomiting. The authors reported that these "were dose-related, were mostly mild to moderate in severity, and were partially mitigated with a lower starting dose" [3].
The same trial found heart rate rising with dose. The increases "peaked at 24 weeks and declined thereafter" [3]. Approved drugs in the same family raise heart rate too; tirzepatide's label reports a rise of a few beats per minute, covered in tirzepatide side effects.
The phase 3 trials found the same pattern, with more detail
Three phase 3 trials have now been published. TRIUMPH-1 enrolled 2,339 adults with obesity and no diabetes for 80 weeks; its summary reports only that "the most common adverse events were gastrointestinal" [4]. The other two give numbers.
TRIUMPH-2 enrolled 1,152 adults with obesity and type 2 diabetes, also for 80 weeks, and tested three dose groups against placebo. Stomach side effects were more common on retatrutide and rose with dose. Low blood pressure and dysesthesia, an unusual skin sensation such as tingling or burning, were also more frequent than on placebo [5].
| Side effect | 4 mg group | 9 mg group | 12 mg group | Placebo |
|---|---|---|---|---|
| Diarrhea | 27% | 34% | 34% | 13% |
| Nausea | 14% | 21% | 28% | 8% |
| Dysesthesia (skin tingling or burning) | 4% | 6% | 7% | 1% |
| Low blood pressure | 1% | 5% | 6% | Under 1% |
| Stopped treatment for side effects or death | 4% | 12% | 8% | 5% |
Bellido 2026, The Lancet; funded by Eli Lilly [5]. Dose groups are the arms the trial tested, not a guide to any dose.
In the 9 mg group, 12% stopped treatment because of side effects or death, more than double the placebo rate. Seven people died during the trial across all groups, including one on placebo; the investigators judged none of the deaths related to the study drug [5]. The authors called the safety profile "generally consistent with other molecules with GLP-1 receptor agonism."
TRANSCEND-T2D-1 tested retatrutide alone in 537 adults with type 2 diabetes for 40 weeks. The most frequent side effects were "generally mild to moderate gastrointestinal events, which subsided over time," 2% to 5% stopped because of side effects vs none on placebo, and no severe low blood sugar was reported [6].
| Trial | People | Length | Main safety findings |
|---|---|---|---|
| Phase 2 (2023), obesity | 338 | 48 weeks | Stomach effects, dose-related, mostly mild to moderate; heart rate rose with dose |
| TRIUMPH-1 (2026), obesity, no diabetes | 2,339 | 80 weeks | Most common side effects were stomach-related |
| TRIUMPH-2 (2026), obesity and type 2 diabetes | 1,152 | 80 weeks | Diarrhea and nausea up to 34% and 28%; more low blood pressure and dysesthesia |
| TRANSCEND-T2D-1 (2026), type 2 diabetes | 537 | 40 weeks | Mostly mild to moderate stomach effects that eased; no severe low blood sugar |
What isn't known yet
- Long-term effects. The longest published trials ran 80 weeks. Nothing yet shows what happens over many years.
- Uncommon side effects. Some problems only show up once a drug is in wide use; approved labels have a whole section for reports like these, and retatrutide has none yet.
- Pregnancy and breastfeeding. There is no label to say what's known; the approved drugs in this family carry pregnancy warnings.
- Heart outcomes. None of the four published trials measured heart attacks or strokes as an outcome.
- The official warnings. Until FDA reviews the full data, there is no list of who shouldn't take it.
Every trial so far was run and paid for by the company developing the drug. That's normal for a new medicine, but it's one more reason the full FDA review matters.
The bigger risk right now: retatrutide sold online
Whatever the trials show, they describe Lilly's study drug, made and tested under trial conditions. What's sold online is something else. FDA says it has warned companies that illegally sold unapproved drugs containing semaglutide, tirzepatide or retatrutide "falsely labeled 'for research purposes' or 'not for human consumption'" [1].
These products have been sold directly to consumers for human use with dosing instructions. The agency urges consumers not to purchase these products which are of unknown quality and may be harmful to their health.[1]
FDA has also warned telehealth companies for marketing retatrutide, ingredient distributors for selling it to compounders, and outsourcing facilities for repackaging it [1]. A vial bought online has no proof of what's in it, how much, or how clean it is, so none of the trial safety data applies to it. FDA's warnings name telehealth companies among the sellers; how to judge a program is covered in GLP-1 telehealth.
Approved drugs with full safety labels
If you want a medicine in this family with a reviewed safety record, two molecules are FDA-approved: tirzepatide, which acts on two of retatrutide's three receptors, and semaglutide. Both come with full prescribing information listing their side effects and warnings. How retatrutide's trial results compare with tirzepatide's is in retatrutide vs tirzepatide.