Retatrutide Side Effects: What Its Clinical Trials Have Actually Reported

Roughly 22,000 searches a month ask some version of "retatrutide side effects," and the honest starting point is one most results skip: there is no FDA-approved retatrutide product, so there is no FDA-approved drug label — and no official side-effect list — for anyone to quote. Our explainer on retatrutide's regulatory status covers why in full: it remains an investigational molecule moving through Eli Lilly's Phase 3 program, with a Biologics License Application not even planned for filing until early 2027.

What we can report, with every figure traced to the document that reported it, is the adverse-event data retatrutide's own clinical trials have actually published or announced. That's real information, and it's worth being precise about its limits: this is safety data collected inside structured trials, on participants monitored by clinical investigators, with a study drug whose content and concentration were verified before a single dose was given. It is not the same thing as a comprehensive real-world safety profile, and it says nothing about the safety of an unregulated, non-FDA-reviewed product bought from an online seller under the retatrutide name.

Phase 2: gastrointestinal events are the dominant, dose-related signal

Retatrutide's earliest published trials both point the same direction. In the 338-participant Phase 2 obesity trial, published in the New England Journal of Medicine, the most common adverse events "were gastrointestinal; these events were dose-related, were mostly mild to moderate in severity, and were partially mitigated with a lower starting dose (2 mg vs. 4 mg)." The same trial reported a second, distinct signal: "dose-dependent increases in heart rate peaked at 24 weeks and declined thereafter¹" — a finding specific to this trial's own reported results, not a generic GLP-1-class side effect assumed to apply here.

The 281-participant Phase 2 type 2 diabetes trial, published in The Lancet, put a number on the gastrointestinal signal: mild-to-moderate GI adverse events — nausea, diarrhea, vomiting, and constipation, grouped together in the trial's own reporting — occurred in 35% of participants across the combined retatrutide arms, ranging from 13% in the lowest-dose (0.5 mg) group up to 50% specifically in the group escalated fastest to 8 mg, against 13% on placebo and 35% on an active dulaglutide comparator arm. The same trial's own conclusion states plainly: "There were no reports of severe hypoglycaemia and no deaths during the study²."

Phase 3, TRIUMPH-1: the fullest per-dose breakdown available so far

TRIUMPH-1 has not yet been published as a full peer-reviewed manuscript — Lilly reported it via topline press release in May 2026, and that distinction is repeated here rather than glossed over. With that flagged clearly, the release itself is unusually specific, breaking adverse-event rates down by the trial's three retatrutide doses against placebo⁴:

The same release reports that discontinuation due to adverse events rose with dose: 4.1% at 4 mg, 6.9% at 9 mg, and 11.3% at 12 mg, compared with 4.9% on placebo⁴ — consistent with the Phase 2 finding that a faster or higher climb in dose carries a real tolerability cost. One detail worth flagging specifically: upper respiratory tract infection appeared at broadly similar rates across every arm, including placebo (11.6%–14.2%), unlike the clearly dose-related GI events — a useful reminder that not every adverse event reported alongside a drug trial is actually caused by the drug being tested.

Phase 3, TRIUMPH-2 and TRIUMPH-3: the same signal in two more populations, plus a cardiovascular finding

A second Lilly topline release, from July 2026, reports adverse-event breakdowns for two more Phase 3 trials — TRIUMPH-2 (adults with obesity and type 2 diabetes) and TRIUMPH-3 (adults with severe obesity and established cardiovascular disease). The pattern repeats: diarrhea, nausea, and constipation were the most common events in both trials, occurring more often at higher doses, with discontinuation due to adverse events reaching 11.6% at 9 mg in TRIUMPH-2 and 13.5% at 12 mg in TRIUMPH-3⁵.

TRIUMPH-3, run specifically in a population with existing cardiovascular disease, also reported a pre-specified analysis of major adverse cardiovascular events (MACE) — the release states these "occurred less frequently than anticipated in both retatrutide and placebo arms," with a hazard ratio of 0.82 (95% CI, 0.55–1.22) for the broader MACE-5 composite (all-cause death, heart attack, stroke, heart failure, or coronary revascularization) and 1.12 (95% CI, 0.64–1.96) for the narrower MACE-3 composite⁵. Both confidence intervals cross 1.0, which means this topline result does not, on its own, establish that retatrutide either raises or lowers cardiovascular event risk relative to placebo — it's a numerically favorable finding reported by the sponsor, not yet a statistically confirmed one, and it should be read with exactly that much weight until (and unless) a full peer-reviewed publication follows.

The one completed, peer-reviewed Phase 3 safety readout: TRANSCEND-T2D-1

Separate from the TRIUMPH obesity program, TRANSCEND-T2D-1 — a 537-participant Phase 3 trial in adults with type 2 diabetes inadequately controlled by diet and exercise — is the one Phase 3 retatrutide trial that has actually been completed and published in full, in The Lancet, as of June 2026. Its own reported safety findings: "the most frequent adverse events with retatrutide were generally mild to moderate gastrointestinal events, which subsided over time." Study-drug discontinuations due to adverse events ran 2–5% with retatrutide against 0% with placebo, no severe hypoglycemia was reported, and two deaths occurred during the study — both in the 4 mg group — which the trial's own reporting describes as "unrelated to the study drug³."

What this data is, and what it isn't

Every number above was collected inside a structured clinical trial: participants were screened for eligibility, monitored on a fixed visit schedule, and had their study drug's identity and dose verified before it was administered. That's a meaningfully different safety context than an unregulated seller's vial. As our companion article on retatrutide's approval status documents directly from FDA's own warning-letter and DailyMed records, products marketed online under the "retatrutide" name today are unapproved, unreviewed by FDA for identity or purity, and in at least one case named by FDA specifically as an unapproved drug sold despite "Research Use Only" labeling. None of the adverse-event data above was collected on any of those products — it describes retatrutide, the molecule, as administered inside Lilly's own trials, under trial-level oversight that a purchase from an unregulated source simply does not have.

The bottom line

Across every retatrutide trial reported so far — two completed Phase 2 studies, one completed and peer-reviewed Phase 3 trial, and two Phase 3 trials so far reported only via topline release — gastrointestinal adverse events (nausea, diarrhea, vomiting, constipation) are the consistent, dose-related signal, discontinuation due to adverse events rises with dose, and no trial to date has reported severe hypoglycemia. Reported deaths in the one completed Phase 3 trial were adjudicated by the trial itself as unrelated to the drug. None of that adds up to an FDA-reviewed safety profile — it's trial-population data, generated under supervision this site cannot verify exists for any product sold outside those trials. For the dosing schedules that produced this adverse-event data, see our companion article on what retatrutide's trials actually tested; for the regulatory status governing all of it, start with retatrutide's approval status.