Retatrutide Side Effects: What the Trials Actually Found

In the published trials of retatrutide, the most common side effects were nausea, diarrhoea, vomiting and constipation. They were mostly mild to moderate and happened mainly while the dose was being stepped up. In the largest trial, 42% of people on the highest dose reported nausea, against 15% on placebo, and 11% stopped treatment because of side effects, against 5%. Average heart rate rose, more at higher doses.¹³⁸⁹

Those figures describe Lilly's drug, given under medical supervision. Retatrutide is not approved anywhere as of 22 October 2026; Lilly says it "cannot be legally sold or marketed for human use" and plans to apply to the FDA in the first quarter of 2027.¹⁰ If you are using a product sold online under its name, the trial data may not describe what you are taking. The section on grey-market products below explains why.

This article gives no dosing information, recommends no products and is not medical advice. For price, approval and comparisons with other drugs, see retatrutide questions answered.

Side-effect rates in seven trials

Each cell shows the placebo group, then the group given the highest dose tested in that trial. The figures come from published papers, Lilly's medical information pages that summarise those papers, ClinicalTrials.gov results and, where marked, company announcements. All were checked on 22 October 2026.

Figures are placebo → highest dose, rounded. ¶Phase 2 "stopped" figures are people who left the study because of an adverse event, as posted on ClinicalTrials.gov; the papers' figures for stopping treatment were not available to us. †From Lilly's July 2026 announcement. ‡Includes deaths; 12% stopped at the middle dose. §As reported by Healio from the Lancet paper.

Different trials enrolled different people, so rows should not be compared directly; our guide to reading a trial results table explains why. TRIUMPH-3 and TRIUMPH-4 have so far been announced only by press release and have not been through peer review (how to check). All the published trials were funded by Eli Lilly, and several authors are Lilly employees.²⁴⁵

Nausea, and why it can be worse after eating

Across the three retatrutide groups in TRIUMPH-1, 29% to 42% of people reported nausea, against 15% on placebo. It was the most frequently reported side effect.⁸ In TRANSCEND-T2D-1, Lilly reports that gastrointestinal events were "mostly mild to moderate in severity, occurred primarily during the dose-escalation period, and subsided over time".⁹

The retatrutide summaries do not explain the link with meals, but the labels of related approved drugs describe the likely mechanism. GLP-1 drugs slow the rate at which the stomach empties, so a meal stays there longer. The Zepbound label states: "Tirzepatide delays gastric emptying. The delay is largest after the first dose and this effect diminishes over time."¹⁹ The Wegovy label says the same of semaglutide.²⁰ Zepbound's most common side effects include nausea, vomiting, indigestion, belching and reflux.¹⁹ Retatrutide also activates the GLP-1 receptor, but its own effect on stomach emptying is not reported in the sources we used.³

Constipation

Constipation affected 24% to 26% of people on retatrutide in TRIUMPH-1, against 11% on placebo.⁸ The summaries give rates, not durations, so there is no trial answer to whether constipation lasting weeks is typical. The Zepbound label lists "severe constipation including fecal impaction" among reports received after approval for tirzepatide and the GLP-1 class.¹⁹ Constipation that does not settle, or comes with severe pain, a swollen belly or vomiting, needs a doctor.

Tiredness

Fatigue was reported by 10% of people on the highest dose in TRIUMPH-1, against 4% on placebo, and by 7% against 3% in TRANSCEND-T2D-1.⁸⁹ The tables do not say how long it lasted or how severe it was. On the Zepbound label, fatigue (a grouping that includes weakness, lethargy and malaise) affected 7% at the highest dose against 3% on placebo.¹⁹

Two other TRIUMPH-1 findings can feel similar. Low blood pressure, including feeling faint on standing, was reported by 8.8% at the highest dose against 0.9% on placebo, more often in people taking blood-pressure medicines. Dizziness was reported by 12.0% against 3.1%.⁸

A racing heart

Like other GLP-1-based drugs, retatrutide raises average resting heart rate.

• In the phase 2 obesity trial, "dose-dependent increases in heart rate peaked at 24 weeks and declined thereafter".¹ At the end of that trial, the highest-dose group was about 6 beats per minute above its starting rate, against 0.4 on placebo.¹⁴
• A 2026 meta-analysis of weight-loss trials found that, averaged across doses, retatrutide raised heart rate by 3.46 beats per minute more than placebo. The figures were 3.35 for semaglutide and 2.05 for tirzepatide, from different trials.¹⁴
• In TRIUMPH-1, severe or serious heart-rhythm or conduction problems affected 0.9% at the highest dose and 0.9% on placebo.⁸ In TRANSCEND-T2D-1, rhythm or conduction problems of any severity affected 1% to 3% on retatrutide and none on placebo.⁹ The two phase 2 trials each recorded one serious case: atrial fibrillation in the diabetes trial, and an ECG change called QT prolongation at the highest dose in the obesity trial.⁶⁷
• A preprint using US health records found heart rate up 4.3 beats per minute at 3 months in trial volunteers and 2.5 in people using non-trial products. It has not been peer-reviewed.¹⁷

Why it happens. For approved GLP-1 drugs, the reason is "incompletely understood". One explanation is that GLP-1 receptors in the heart's pacemaker tissue are switched on directly; another is a shift in the nervous-system balance that controls heart rate.¹⁴ Whether retatrutide's glucagon component adds to this is not settled. In a lab study, retatrutide made strips of human heart muscle, taken during heart surgery, contract more strongly through all three of its receptors; that was tissue in a dish and did not measure heart rate.¹⁵ The authors of a social media study suggested glucagon activity might explain some symptoms users reported, including heart-rate complaints, but did not test this.¹⁶ Our guide to multi-receptor agonists explains the three targets.

If you also have anxiety

The trials cannot answer this directly. They did not report results separately for people with anxiety, and they excluded some groups. The phase 2 obesity trial excluded people with a heart attack, stroke or heart-failure admission in the previous 3 months, uncontrolled high blood pressure, or "a major problem with depression or other mental illness within the last 2 years".⁶ TRIUMPH-1 excluded people with a recent heart attack, stroke, unstable angina or heart-failure admission, and those with the most severe heart failure.⁸

TRIUMPH-3 deliberately enrolled people with established heart or artery disease, though not anyone with a heart attack, stroke or similar event in the previous 90 days. Lilly reported a hazard ratio of 1.12 for heart attack, stroke or cardiovascular death against placebo, with a wide confidence interval (0.64 to 1.96), so the trial could not show a clear difference either way.¹⁰²³ Whether the heart-rate rise matters for heart attacks and strokes is what TRIUMPH-Outcomes is testing; its estimated primary completion date is February 2029.²²

A fast heartbeat has many possible causes. The Zepbound label, for example, lists a fast heartbeat and anxiety among the signs of low blood sugar.¹⁹ The Wegovy label tells patients to report "palpitations or feelings of a racing heartbeat while at rest".²⁰ If you have a heart condition or an anxiety disorder, or notice a fast heartbeat that persists, talk to a doctor.

Serious side effects in the largest trial

Serious adverse events were reported in 10.5% of people on the highest dose in TRIUMPH-1 and 5.5% on placebo.⁸ Specific problems (highest dose vs placebo)⁸:

• Pancreatitis confirmed by an expert committee: 0.7% vs 0.3%.
• Severe or serious gallbladder problems: 0.7% vs 0.7%.
• Severe or serious stomach and bowel events: 5.2% vs 1.4%.
• Deaths: one at the highest dose, three at the middle dose, two on placebo. Lilly lists the causes, including road accidents. The death at the highest dose was in a person with known heart failure who had been off the drug for 189 days.

In TRIUMPH-2, investigators judged none of the seven deaths related to the study drug.⁴ Our guide to peptide side effects explains how to read adverse-event counts.

Less familiar effects

Dysesthesia, an abnormal skin sensation that Lilly's table labels "skin burning sensation and related AEs", is the effect that sets retatrutide apart. It affected 12.5% of people at the highest dose in TRIUMPH-1, against 0.9% on placebo, and 20.9% in TRIUMPH-4, against 0.7%.⁸¹¹ Lilly says it was mostly mild and rarely led people to stop treatment; how long episodes lasted has not been reported.¹¹ For comparison, the Zepbound label lists dysesthesia in 0.4% of people at its highest dose.¹⁹

Urinary tract infections were reported in 7.5% to 8.7% of people on retatrutide in TRIUMPH-1, against 5.3% on placebo, mostly in women.⁸

Hair loss did not appear in the trial tables we could access, which listed only common effects. With tirzepatide, the label says hair loss was "associated with weight reduction".¹⁹ See whether GLP-1 drugs cause hair loss.

Why grey-market users may have different experiences

Products sold online as retatrutide are not Lilly's drug, and their identity, purity and dose cannot be verified.¹⁰¹⁶ See our report on grey-market retatrutide and what "research use only" means. Three recent sources point to a gap between trial results and non-trial use; none can show what caused what.

A JAMA Network Open study of posts by 13,589 Reddit users who described using retatrutide found the most common complaints were increased appetite, fatigue and increased energy, followed by nausea, food cravings, insomnia and a raised heart rate. Stomach symptoms were less dominant than in the trials. The authors call the results "hypothesis-generating" and say the analysis "cannot assess incidence, causality".¹⁶

A preprint using US health records found that people using non-trial products lost 7.2% of body weight at 6 to 12 months, against 15.5% for trial volunteers seen in routine care. It has not been peer-reviewed.¹⁷

CBS News reported in October 2026 on two reports of deaths sent to FDA in people who had used non-trial products. FDA declined to say whether it had verified them, and the reports do not prove the products caused the deaths.¹⁸

See also whether online peptides are safe to inject.

When to seek medical help

Retatrutide has no approved label yet. The closest approved relative is Zepbound (tirzepatide), a different drug that acts on two of the same three receptors. Its label and patient guide warn about the following¹⁹:

• Possible pancreatitis: "severe pain in your stomach area (abdomen) that will not go away, with or without nausea or vomiting", sometimes spreading to the back. Seek urgent care.
• Dehydration: "Diarrhea, nausea, and vomiting may cause a loss of fluids (dehydration) which may cause kidney problems." Tell a doctor right away if these do not go away.
• Gallbladder problems: pain in the upper stomach, fever, yellowing of the skin or eyes, or clay-coloured stools.

Chest pain or fainting is an emergency: go to an emergency room or call emergency services. A fast heartbeat at rest that does not settle needs prompt medical assessment. If you are using any product sold as retatrutide, tell the clinician exactly what it was and where it came from. You can report problems to FDA MedWatch or your national regulator.²¹ The main legal route to the real drug is a clinical trial; see how to join a weight-loss drug trial.