Retatrutide Side Effects: What the TRIUMPH Trials Report, Dose by Dose

Retatrutide’s side effects in its trials were mainly nausea, diarrhea, constipation and vomiting, and they climbed with the dose. Two findings stood out against placebo: an altered skin sensation called dysesthesia, reported in up to one in eight people on the top doses of its largest trial, and more low blood pressure in people with diabetes.^[2][3] On the 12 mg dose, 11.3% of people in that trial stopped because of side effects, against 4.9% on placebo, according to Lilly.^[2] Retatrutide is not approved anywhere. It has no label, and no regulator has yet reviewed its safety.^[11][12]

Retatrutide side effects by dose: TRIUMPH-1

TRIUMPH-1 is the largest retatrutide trial published so far: 2,339 adults with obesity and without diabetes, randomized to 4, 9 or 12 mg weekly or to placebo for 80 weeks.^[1] The summary of its NEJM paper names gut problems as the leading adverse events and gives no per-dose breakdown.^[1] The per-dose rates below are from Lilly’s topline release.^[2]

Read across a row and the dose effect is plain. Vomiting goes from about one in ten on 4 mg to one in four on 12 mg. Nausea on the top dose was nearly three times the placebo rate. The 4 mg arm is the exception at the bottom of the table: its dropout rate for side effects was slightly below placebo’s.^[2] How the arms were stepped up to their target doses is on retatrutide dosing in the trials.

The earlier phase 2 trial, 338 adults over 48 weeks, gives the same shape in peer-reviewed form: stomach and bowel complaints rose with dose, most were not severe, and starting at 2 mg rather than 4 mg partly reduced them.^[7]

Side effects in people with type 2 diabetes

TRIUMPH-2 is the one phase 3 obesity trial with a full peer-reviewed safety summary. Its 1,152 participants had diabetes on top of obesity, and 92% were already taking oral diabetes medicine.^[3]

Two things in that table stand out. Hypotension, which the authors list alongside dysesthesia as more frequent on retatrutide, reached 6% on 12 mg against under 1% on placebo. And the dropout curve does not rise in a straight line: more people quit on 9 mg than on 12 mg.^[3] The paper calls the overall profile “generally consistent” with other drugs that act on the GLP-1 receptor.^[3]

Two smaller diabetes trials add little alarm. In TRANSCEND-T2D-1, 537 adults whose diabetes was managed by diet and exercise alone, gut side effects were generally mild to moderate and subsided over time; 2% to 5% stopped for side effects against 0% on placebo, and no severe hypoglycemia occurred.^[6] In the phase 2 diabetes trial, gut side effects affected 35% of people on retatrutide overall, ranging from 13% on 0.5 mg to 50% on 8 mg with fast escalation, against 13% on placebo and 35% on dulaglutide. There was no severe hypoglycemia and no death.^[8]

How many people stopped retatrutide because of side effects

Discontinuation for adverse events is the bluntest tolerability measure a trial reports. Across the program it ranged widely. The TRIUMPH-3 and TRIUMPH-4 figures are from Lilly releases.^[4][5]

TRIUMPH-4’s 18.2% on 12 mg is the highest figure in the program. Lilly said those dropouts were “highly correlated with baseline BMI” and included people who stopped for what it called perceived excessive weight loss. Among participants who started with a BMI of 35 or more, the rate on 12 mg fell to 12.1%, against 4.8% on placebo.^[5] People with less weight to lose quit more often, and some quit because the drug worked further than they wanted.^[5]

Dysesthesia: the skin side effect

Dysesthesia means an abnormal, usually unpleasant skin sensation: burning, prickling, tingling or numbness without an obvious cause. Every retatrutide trial that reported it found it more often on the drug than on placebo.^[3][5]

In Lilly’s account, most cases were mild or moderate, cleared up while people kept taking the drug, and seldom led anyone to quit.^[2][5] The one-in-five figure from TRIUMPH-4 comes from a 445-person trial that has not been published.^[5] The effect is not unique to retatrutide: the Wegovy label reports dysesthesia in 22% of people on the 7.2 mg injection against 0.3% on placebo, and in 5% on the 25 mg semaglutide tablet against none on placebo.^[14] More on that in Wegovy pill side effects. No published retatrutide data yet explain what causes it, or show whether it fully reverses once the drug is stopped.

Heart rate and the heart

Retatrutide raises resting heart rate. The phase 2 obesity paper reported dose-dependent increases that peaked at 24 weeks and declined after that.^[7] A 2026 meta-analysis of trials in people without diabetes put the average rise on retatrutide at 3.46 beats per minute over placebo (95% CI 1.74 to 5.18), close to semaglutide’s 3.35 and above tirzepatide’s 2.05.^[9] That analysis was published in January 2026, before any phase 3 retatrutide results, so its retatrutide estimate rests on earlier-phase trials.

Arrhythmia is a different question, and the published record does not answer it. None of the retatrutide trial abstracts reports a rate of irregular heart rhythm, and no cardiovascular outcome trial has reported. Counting rare rhythm problems against a placebo arm is one of the jobs of the FDA’s review, and that review has not started. For what a faster pulse means on the approved drugs, see GLP-1 heart rate and palpitations.

Low blood sugar, gallbladder and pancreatitis

• Low blood sugar. Neither diabetes trial that reported it found severe hypoglycemia: none in TRANSCEND-T2D-1 and none in phase 2.^[6][8] Those participants were managed with diet and exercise, or metformin in phase 2. No published retatrutide summary reports the risk on top of insulin or a sulfonylurea.
• Gallbladder disease. No published retatrutide trial summary gives a gallstone rate. A 2026 review of the drug class judges gallstones a probable class effect, driven largely by rapid weight loss.^[10] Retatrutide produces more weight loss than the approved drugs, so this is one to watch in the full data.
• Pancreatitis. Also unreported in the retatrutide summaries. The same review found randomized trial data reassuring for the class as a whole, with no excess risk shown.^[10] The approved drugs’ labels and trials are covered in GLP-1 pancreatitis and gallbladder risk.
• Low blood pressure. Up to 6% on 12 mg in TRIUMPH-2, against under 1% on placebo.^[3]
• Deaths. TRIUMPH-2 recorded six deaths across the retatrutide arms and one on placebo; TRANSCEND-T2D-1 recorded two, both on 4 mg. Investigators judged every one unrelated to the study drug.^[3][6]

The same review concludes that early data on retatrutide suggest a gut side-effect profile similar to the approved drugs, while noting that the evidence is still limited.^[10] How the approved drugs rank against one another is on which GLP-1 has the worst side effects.

Is retatrutide safe?

Its safety has not been established, because no regulator has reviewed it. The FDA says retatrutide has “not been found safe and effective for any condition,” and Lilly says no regulator anywhere has approved a medicine containing it.^[11][12] Lilly’s stated target for filing with the FDA is Q1 2027.^[13] Until that review is done there is no label, no list of warnings, no approved dose and no guidance on who should not take it.

What the trials do show is tolerability, not long-term safety. The longest results run 80 weeks. They describe side effects that are common, mostly gastrointestinal, dose-related and usually mild, plus dysesthesia and low blood pressure more often than on placebo. They cannot yet describe what happens over five or ten years, or the rate of rare events, which takes tens of thousands of patient-years to detect. For the efficacy side of the trade-off, see what the evidence shows; for how it sets against an approved drug, retatrutide vs Zepbound.

Why trial side effects do not describe gray-market retatrutide

Every figure above describes trial-grade drug, given at a known dose, raised in four-week steps from 2 mg, and watched by investigators who recorded each event.^[5] Vials sold online as “research peptides” share none of those conditions. The FDA warns that products sold under research-only or not-for-human-use labels are of unknown quality and may harm the people who inject them.^[11] A vial that holds more drug than its label says pushes the buyer up the dose rows of the tables above. One that holds an impurity adds a risk no trial measured. And with no prescriber involved, someone who develops dysesthesia or low blood pressure has no one to report it to.

The market itself, and why a good trial result increases demand for it, is covered in retatrutide’s phase 3 and the gray market. The legal status, FDA letters and online prices are on retatrutide price and release date. Anyone who has already used it and has a reaction should tell a clinician what they took; the FDA asks for such reports through MedWatch.^[11]