Retatrutide Side Effects: What the Phase 3 Trials Actually Reported
Retatrutide is Eli Lilly's investigational once-weekly GIP, GLP-1 and glucagon receptor agonist. It is not FDA-approved, and Lilly says it plans to submit it to FDA in Q1 2027. Everything below comes from Lilly's published TRIUMPH trial results, not from forums. It is educational, not a dosing guide. The 4 mg, 9 mg and 12 mg figures are the doses studied in the trials, not recommendations.
Quick answer: in Phase 3, the most common side effects were gastrointestinal (nausea, diarrhea, constipation, vomiting). The one that surprised people was dysesthesia, an abnormal skin sensation that Lilly describes as "skin burning sensation and related" events. It was more common at higher doses, mostly mild to moderate, and most people kept taking the drug.
Most Common Side Effects by Dose (TRIUMPH-1)
TRIUMPH-1 enrolled 2,339 adults with obesity or overweight and no diabetes, for 80 weeks. Lilly reported these rates (retatrutide 4 mg / 9 mg / 12 mg vs placebo):
| Side effect | 4 mg | 9 mg | 12 mg | Placebo |
|---|---|---|---|---|
| Nausea | 28.6% | 38.4% | 42.4% | 14.8% |
| Diarrhea | 25.2% | 34.1% | 32.0% | 13.5% |
| Constipation | 23.8% | 25.9% | 26.1% | 10.9% |
| Vomiting | 10.6% | 22.8% | 25.3% | 4.8% |
| Dysesthesia | 5.1% | 12.3% | 12.5% | 0.9% |
| Urinary tract infection | 7.5% | 8.8% | 8.4% | 5.3% |
| Stopped due to side effects | 4.1% | 6.9% | 11.3% | 4.9% |
In the trial, everyone started at 2 mg and stepped up every four weeks to their assigned dose. That slow escalation is part of how the trials managed GI effects, and it's one reason forum "fast titration" stories don't compare to trial data.
What Is Retatrutide Dysesthesia?
Dysesthesia means an unpleasant or abnormal sense of touch: burning, tingling, prickling or skin sensitivity without an obvious cause. In the TRIUMPH program, Lilly grouped it as "skin burning sensation and related" adverse events.
Lilly's medical information page summarizes rates across all four completed Phase 3 trials (rounded):
| Trial (population) | Placebo | 4 mg | 9 mg | 12 mg |
|---|---|---|---|---|
| TRIUMPH-1 (obesity, no diabetes) | 1% | 5% | 12% | 13% |
| TRIUMPH-2 (obesity + type 2 diabetes) | 1% | 4% | 6% | 7% |
| TRIUMPH-3 (obesity + heart disease) | 1% | not studied | 6% | 6% |
| TRIUMPH-4 (obesity + knee osteoarthritis) | 1% | not studied | 9% | 21% |
What the published data say about it:
- It was dose-related: more common at 9 mg and 12 mg than 4 mg.
- It was mostly mild to moderate. In TRIUMPH-1, Lilly said most events resolved during treatment and most participants continued.
- In TRIUMPH-4, Lilly said dysesthesia "rarely led to treatment discontinuation," even though about 1 in 5 people on 12 mg reported it.
What the data don't tell you yet: the exact mechanism, how long individual episodes last, or whether it's more likely in certain people. If you notice burning or tingling skin, log when it started, where it is, and how long it lasts, and bring that to a clinician. New numbness, weakness, or symptoms that spread need prompt medical attention whatever you're taking.
GI Side Effects: Why They Cluster Early
Nausea, diarrhea, constipation and vomiting are the classic incretin side effects, and retatrutide follows the same pattern. In TRIUMPH-1, GI events were the most common reason people stopped treatment (2.2% to 4.6% in the retatrutide groups vs 1.2% on placebo, per Lilly's ADA 2026 presentation).
Clinicians usually ask about:
- Timing: symptoms in the days after a dose increase vs steady-state weeks
- Hydration: vomiting and diarrhea can dehydrate you, which matters for kidney function
- Severe or persistent abdominal pain: this needs urgent evaluation, since pancreatitis and gallbladder problems are warnings on approved drugs in this class
Low Blood Pressure and Injection-Site Reactions
Lilly's TRIUMPH-1 presentation and medical information page listed these as adverse events of special interest (4 mg / 9 mg / 12 mg vs placebo):
- Hypotension, orthostatic hypotension or decreased blood pressure: 2.7% / 5.3% / 8.8% vs 0.9%. Lilly noted it was more common in people already taking blood pressure medicines.
- Injection-site reactions: 5.5% / 8.9% / 12.2% vs 3.8%.
If you take blood pressure medicine, dizziness on standing is worth mentioning to your prescriber. Weight loss itself can change how much blood pressure medication you need.
Fatigue, Mood and "Anhedonia" Searches
Searches like "does retatrutide make you tired" and "retatrutide anhedonia" are rising. Lilly's topline releases don't list fatigue or mood changes among the most common adverse events, and we haven't found trial data that quantify them for retatrutide. Treat online reports as anecdotes: they can reflect eating much less, poor sleep, dehydration, or an unverified product. If your mood drops or you lose interest in things you used to enjoy, talk to a clinician. Don't wait for a trial to confirm it.
Gray-Market Vials Add a Different Kind of Risk
Every number above comes from trials using Lilly's manufactured drug, at known doses. Retatrutide sold online is a different product. In September 2026, Australia's TGA tested a vial sold as retatrutide after a patient was hospitalized with a torn oesophagus from uncontrollable vomiting. The vial contained no retatrutide. It held undeclared semaglutide at about 8 times the level in approved semaglutide products. FDA has said retatrutide can't be legally compounded under federal law.
