Retatrutide side effects: how common they are and what the trials saw

The most common side effects of retatrutide are stomach problems: nausea, diarrhea, constipation and vomiting, rising with dose. One signal stands out, a burning or altered skin sensation called dysesthesia. Retatrutide is not FDA-approved, so it has no label and no official side-effect list. Everything here comes from Lilly’s trials: the phase 2 papers from 2023, the phase 3 papers published in September and October 2026, and Lilly’s topline press releases, which are labeled as such because they have not all been peer reviewed.

Retatrutide side effects at a glance

The phase 3 numbers, trial by trial

TRIUMPH-1: obesity without diabetes, 80 weeks

Source: Lilly press release, May 21, 2026 (topline). 2,339 people randomized. Lilly said the types of side effects “were generally consistent with trials of other incretin-based therapies.” The 4 mg dose, reached with one step up from 2 mg, had a lower discontinuation rate than placebo.

TRIUMPH-2: obesity with type 2 diabetes, 80 weeks

Sources: Lancet 2026 (diarrhea, nausea, dysesthesia, hypotension, discontinuation) and Lilly press release, July 23, 2026 (constipation, appetite, vomiting). 1,152 people randomized. Seven participants died during the trial (two on 4 mg, three on 9 mg, one on 12 mg, one on placebo); investigators judged all deaths unrelated to the study drug. Low blood pressure matters here, because this population often also takes blood pressure medicines.

TRIUMPH-3: severe obesity with heart disease, 80 weeks

Source: Lilly press release, July 23, 2026 (topline). 1,949 people randomized. High blood sugar was reported less often on retatrutide (3.9% and 3.1%) than on placebo (13.4%).

TRIUMPH-4: obesity with knee osteoarthritis, 68 weeks

Source: Lilly press release, December 11, 2025 (topline). 445 people randomized, 84% with a BMI of 35 or more. Lilly said the discontinuation rates “were highly correlated with baseline BMI and included discontinuations for perceived excessive weight loss.” Among participants starting at a BMI of 35 or more, 8.8% and 12.1% stopped because of side effects on 9 and 12 mg, versus 4.8% on placebo. Overall discontinuation for any reason was similar between the drug and placebo arms.

TRANSCEND-T2D-2: type 2 diabetes, against Ozempic, 80 weeks

This open-label trial is the first time retatrutide’s side effects have been measured directly against another incretin drug.

Source: Lilly press release, October 28, 2026 (topline, not peer reviewed). 1,298 people randomized. The stomach rates sit close to Ozempic’s, while more people stopped retatrutide. In the shorter 40-week TRANSCEND-T2D-1 trial, 2.2% to 5.1% stopped because of side effects, against none on placebo, and dysesthesia was 2.3% to 4.5%, against none on placebo.

Dysesthesia: why does retatrutide cause skin burning?

The cause has not been explained in the published summaries. Dysesthesia means an abnormal, often unpleasant sensation on the skin, and Lilly’s medical information service defines it for the TRIUMPH trials as “a skin burning sensation and related adverse events.” It is the retatrutide side effect most often asked about.

Source: Lilly Medical, last reviewed August 18, 2026. The signal was already visible in phase 2, where “cutaneous hyperesthesia and skin sensitivity” (heightened skin sensitivity) was reported in 7% of people on retatrutide and 1% on placebo. The phase 2 authors wrote that these cases “were mild to moderate in severity and did not lead to discontinuation,” and “did not appear to be related to the magnitude or rate of weight loss.” In phase 3, Lilly says events were “generally mild to moderate, the majority resolved during treatment, and most participants continued taking retatrutide.” TRIUMPH-4 described them as rarely leading to stopping the drug.

Retatrutide is not alone here. The Wegovy label reports dysesthesia in 22% of people on the 7.2 mg dose, against 6% on 2.4 mg and 0.3% on placebo. The Zepbound label reports it in 0.2% to 0.4%, against 0.1% on placebo.

When do retatrutide side effects start, and do they fade?

Stomach side effects mostly arrive during dose increases and then ease. In the phase 2 obesity trial they happened mainly during escalation, and Lilly says most phase 3 events resolved during treatment. Day-by-day timing for retatrutide has not been published, so the closest guide is its drug class.

Source: Gorgojo-Martinez et al. 2022, an expert consensus on GLP-1 drugs, not a retatrutide study. Retatrutide’s trial ladder keeps rising until week 16 at the top dose, so the stretch with new dose increases runs longer than the first few weeks.

What helps with nausea, vomiting and constipation?

No study has tested remedies in people taking retatrutide. The practical advice below comes from that same GLP-1 consensus.

• Eating. Smaller portions, eaten slowly, stopping at the first feeling of fullness. Bland, low-fat, easy-to-digest food. Not lying down right after a meal, and not eating close to bedtime.
• Nausea. Avoiding strong smells. Taking small sips of clear fluids.
• Vomiting. Paying particular attention to fluids, with small amounts of food more often.
• Diarrhea. Plenty of fluids. Cutting back for a while on dairy, coffee, alcohol, soft drinks and sweeteners ending in “ol,” such as sorbitol.
• Constipation. Enough fiber, plenty of water and regular physical activity.

Dose changes are a clinical decision. In a trial, your study team decides whether to hold or adjust a dose, so tell them before you change anything.

Red flags: when to call and when to go to the ER

Retatrutide has no label, but the labels of approved drugs in this class describe the serious problems to watch for. The phase 2 obesity trial recorded one case of acute pancreatitis, in the 12 mg group.

If you may have injected too much, call Poison Help at 1-800-222-1222 or go to an emergency room. Tell any clinician exactly what you injected, including the vial label. You can report a problem to the FDA’s MedWatch program at 1-800-FDA-1088.

Heart rate and heart rhythm

Both glucagon and GLP-1 can make the heart beat faster, so heart rate is a watched measure for a drug that activates both receptors. In the 2023 phase 2 obesity trial, heart rate “increased in a dose-dependent manner with retatrutide up to 24 weeks and then declined thereafter,” and the authors said the increases “were similar to those reported for GLP-1 receptor agonists.” For comparison, the Zepbound label reports an average rise of 1 to 3 beats per minute.

The same trial tracked cardiac arrhythmia (supraventricular arrhythmias and conduction disorders) as an event of special interest:

Groups were small, 33 to 70 people each, so single cases move these percentages a lot. The authors reported the arrhythmias as mild to moderate, apart from one severe event, a prolonged QT interval, in a participant also taking the anti-nausea drug ondansetron.

Phase 3 heart rate figures are not in Lilly’s press releases or the published abstracts, and we could not verify them. TRIUMPH-3 enrolled people with established cardiovascular disease, and events were fewer than expected in both arms: 44 five-part MACE events on retatrutide (all-cause death, heart attack, stroke, heart failure event or coronary revascularization) versus 52 on placebo, a hazard ratio of 0.82 with a 95% confidence interval of 0.55 to 1.22. For the narrower three-part MACE, there were 27 versus 23 events, hazard ratio 1.12 (0.64 to 1.96). Both intervals cross 1, which means the trial cannot show whether retatrutide raises or lowers heart risk.

Other findings from phase 2

The phase 2 obesity trial (338 people, 48 weeks) has the most detailed published safety table so far:

• Any side effect: 73% to 94% on retatrutide versus 70% on placebo.
• Stopped because of side effects: 6% to 16% on retatrutide versus none on placebo, with the highest rate (16%) on 12 mg.
• Gastrointestinal events were mostly mild to moderate and were the most common reason for stopping. Starting at 2 mg rather than 4 mg cut nausea from 36% to 18% at 4 mg and from 60% to 17% at 8 mg.
• One case of acute pancreatitis occurred. Other rises in pancreatic enzymes were without symptoms.
• Liver enzyme ALT rose above three times normal in 1% of people on retatrutide; average liver enzymes were unchanged or lower at 48 weeks.
• Fourteen people’s BMI fell to 22 or lower (13 on retatrutide); eight had protocol-driven dose reductions. No clinically significant low blood sugar was reported.

In the phase 2 diabetes trial, mild-to-moderate stomach side effects were reported by 35% of people on retatrutide overall, from 13% on 0.5 mg to 50% in the 8 mg group that started at 4 mg, against 13% on placebo and 35% on dulaglutide 1.5 mg. There was no severe low blood sugar.

Does retatrutide cause hair loss?

Hair loss does not appear among the most common events in Lilly’s phase 3 summaries, but those lists are short, so that does not rule it out. The Foundayo label reports it in 4% to 5% of people versus 2% on placebo, linked to weight loss. Full phase 3 tables should settle the question for retatrutide.

How this compares with tirzepatide

Cross-trial comparisons are unreliable: populations, trial lengths and even placebo rates differ. With that caveat, the pooled Zepbound weight-management trials on the tirzepatide label show nausea of 25% to 29% versus 8% on placebo and discontinuation for side effects of 4.8% to 6.7% versus 3.4%. TRIUMPH-1’s top dose shows more nausea and more discontinuation, but its placebo group also reported nausea nearly twice as often. The tirzepatide side effects page has the full label table, and tirzepatide vs retatrutide sets the two programs side by side. TRIUMPH-5, a direct comparison, is listed as completing its primary phase in November 2026.

What the trials did about side effects

The trials managed side effects through the protocol, and a reader cannot copy that. Everyone started at 2 mg and moved up one step every four weeks, a ladder designed after phase 2 showed a gentler start reduced nausea and vomiting (see retatrutide dosing). Investigators could interrupt or modify doses, and phase 2 required dose reductions when BMI fell too low. Phase 2 also screened out people with a history of pancreatitis, symptomatic gallbladder disease, or medullary thyroid cancer or MEN 2 in themselves or their family. None of those safeguards exist for someone injecting a product bought online.

Products sold online are a different risk

The tables above describe Lilly’s study drug at known doses. Retatrutide sold as a “research” peptide is an unapproved new drug, illegal to use on people, with no verified identity, purity or strength. A 2026 report in BMJ Case Reports described a woman in her mid-20s hospitalized with five days of persistent nausea, repeated vomiting and diarrhea that did not respond to four different anti-nausea medicines, after four weekly injections of an unverified product said to contain retatrutide, given in a wellness setting by a person without medical training. She recovered within a week.

A research letter in JAMA Network Open (October 2026) analyzed Reddit posts from 13,589 people who said they were taking retatrutide, many likely using unapproved products. Among the 7,823 who described a symptom, the most common were increased appetite (17.1%), fatigue (15.2%), nausea (11.6%), food cravings (7.3%), insomnia (6.1%) and raised heart rate (4.8%). The authors call this hypothesis-generating: it cannot show how often anything happens or whether the drug caused it, and contents, dose and other drugs could not be verified. It does show that people using these products report a pattern that differs from the trials, and that nobody can yet explain why. The FDA status tracker follows retatrutide’s path to an FDA decision.