Research material and lot documents.
Retatrutide side effects in the phase 2 and phase 3 papers were mostly gastrointestinal. This page repeats what each paper's authors reported for that trial's participants and gives no advice on any person's use. The retatrutide trials page gives each trial's design and efficacy results.
Diarrhea occurred in 27% to 34% of participants across the retatrutide groups, against 13% with placebo. Nausea occurred in 14% to 28%, against 8%. Hypotension (1% to 6%, against under 1%) and dysesthesia (4% to 7%, against 1%) were also more frequent with retatrutide. Source: 2026 Lancet, PMID 42810372.
The authors described the most frequent adverse events as generally mild to moderate gastrointestinal events that subsided over time, and they reported no severe hypoglycemia. Source: 2026 Lancet, PMID 42250575.
The abstract states that the most common adverse events were gastrointestinal. Source: 2026 New England Journal of Medicine, PMID 42814954.
The authors reported that gastrointestinal events were the most common, mostly mild to moderate in severity, and that increases in heart rate peaked at 24 weeks and declined afterward. Source: 2023 New England Journal of Medicine, PMID 37366315.
Mild-to-moderate gastrointestinal events, including nausea, diarrhea, vomiting and constipation, were reported by 67 of 190 participants (35%) in the retatrutide groups, 6 of 45 (13%) with placebo and 16 of 46 (35%) with dulaglutide. The authors reported no severe hypoglycemia and no deaths. Source: 2023 Lancet, PMID 37385280.
Lilly's topline report for TRIUMPH-4 (adults with obesity or overweight and knee osteoarthritis, 68 weeks) gave dysesthesia in 8.8% and 20.9% of the two retatrutide groups, against 0.7% with placebo. Lilly described these events as generally mild and rarely leading to discontinuation. For TRIUMPH-3 (adults with severe obesity and established cardiovascular disease, 80 weeks) Lilly gave dysesthesia in 6.4% and 6.4% against 1.3%. Neither trial has a results paper listed in PubMed. Sources: Lilly news release of December 11, 2025 and Lilly news release of July 23, 2026.
TRIUMPH-2: permanent discontinuation because of adverse events or death was 4% to 12% across the retatrutide groups and 5% with placebo.
TRANSCEND-T2D-1: discontinuation because of adverse events was 2% to 5% with retatrutide and 0% with placebo.
TRIUMPH-4, sponsor topline: 12.2% and 18.2% in the two retatrutide groups, against 4.0% with placebo. Lilly reported that these rates correlated with baseline BMI and included discontinuations for perceived excessive weight loss.
TRIUMPH-3, sponsor topline: 9.8% and 13.5%, against 4.8%.
Sources: PMID 42810372, PMID 42250575, Lilly news release of December 11, 2025 and Lilly news release of July 23, 2026.
In TRIUMPH-2, six participants in the retatrutide groups and one in the placebo group died, and the investigators judged every death unrelated to the study intervention. In TRANSCEND-T2D-1, two deaths occurred, both in a retatrutide group and both unrelated to the study drug. The phase 2 type 2 diabetes trial reported no deaths. Sources: PMID 42810372, PMID 42250575 and PMID 37385280.
Each trial selected participants by its own entry criteria. The TRIUMPH-5 record on ClinicalTrials.gov, for example, excludes people with a personal or family history of medullary thyroid carcinoma or multiple endocrine neoplasia syndrome type 2, a history of chronic or acute pancreatitis, a heart attack, stroke or coronary revascularization within 90 days before screening, and New York Heart Association class IV heart failure. The rates above come from participants who met criteria of this kind.
A 2026 BMJ Case Reports article described a woman in her mid-20s with persistent nausea, vomiting and loose stools after four weekly treatments with an unverified product reported to contain retatrutide. The authors reported that her symptoms resolved within a week and rated the causality as probable (Naranjo score 7). Because the product was unverified, the report cannot tie her symptoms to retatrutide itself. Source: 2026 BMJ Case Reports, PMID 42843917.
A separate 2026 analysis of three products sold as retatrutide (PMC13445994) measured retatrutide content from 51.3% to 190.0% of the labelled amount. It is summarized on the retatrutide research peptide page. FDA's statements about unapproved retatrutide are quoted on the retatrutide FDA approval page.