What Are the Side Effects of Retatrutide?

Overview of Side Effects

The safety profile of retatrutide has been evaluated in Phase 1 and Phase 2 clinical trials. The most commonly reported adverse events are gastrointestinal (GI) in nature, which is consistent with other incretin-based therapies such as semaglutide and tirzepatide.

The key GI side effects include nausea, diarrhea, vomiting, constipation, and decreased appetite. These effects are related to retatrutide's mechanism of action on GLP-1 and GIP receptors, which affect gastric motility and satiety signaling.

Side Effect Rates from Phase 2 Data

In the Phase 2 trial published in The New England Journal of Medicine (2023), side effects were dose-dependent. The following rates were observed across retatrutide dose groups compared with placebo:

• Nausea: Reported in approximately 16% to 45% of participants depending on dose, compared with roughly 8% for placebo. Higher doses (8 mg and 12 mg) were associated with higher rates.
• Diarrhea: Occurred in about 14% to 34% of participants across dose groups, versus approximately 10% with placebo.
• Vomiting: Ranged from approximately 6% to 14% across dose groups in Phase 2, compared with about 2% for placebo. Phase 3 TRIUMPH-4 data showed rates of approximately 21% at the 12 mg dose.
• Constipation: Reported in roughly 6% to 12% of participants, similar to or slightly above placebo rates.
• Decreased appetite: Noted in up to 10% of participants at higher doses, which is considered both a side effect and a component of the therapeutic mechanism.

Dose Dependence and Dose Escalation

Side effects were clearly dose-dependent — participants receiving lower doses (1 mg or 4 mg) experienced substantially fewer GI symptoms than those receiving higher doses (8 mg or 12 mg). Importantly, gradual dose escalation (starting at a lower dose and increasing over weeks) was used in the trials to reduce the severity and incidence of GI side effects.

Most GI symptoms were most prominent during the early weeks of treatment and during dose escalation periods. Symptoms generally decreased in both frequency and severity as treatment continued.

Severity and Discontinuation Rates

The majority of GI side effects were classified as mild to moderate. Discontinuation rates due to adverse events were relatively low across dose groups in Phase 2, though specific discontinuation rates varied by dose level. Serious adverse events were uncommon and did not show a clear dose-dependent pattern.

Phase 3 TRIUMPH-4 Safety Data

Phase 3 TRIUMPH-4 data (December 2025) provided a larger-scale safety assessment and confirmed the dose-dependent GI side effect pattern. Updated GI rates at the 12 mg dose included nausea (43.2%), diarrhea (33.1%), and vomiting (~21%).

TRIUMPH-4 also identified a new safety signal: dysesthesia — abnormal sensations including tingling, numbness, or burning. Dysesthesia was reported in 0.7% of the placebo group, 8.8% of the 9 mg group, and 20.9% of the 12 mg group. This signal was not prominent in Phase 2 data and is thought to be linked to glucagon receptor activity. Dysesthesia was generally mild and infrequently led to treatment discontinuation.

Other Reported Effects

Beyond GI symptoms, some participants experienced injection site reactions, increased heart rate (a class effect seen with GLP-1 receptor agonists), and transient increases in certain liver enzymes. These findings are consistent with the broader class of incretin-based medications and were generally not clinically significant in Phase 2 data.

Important Caveats

While Phase 3 TRIUMPH-4 data have expanded the safety picture — including higher GI rates at the 12 mg dose and the new dysesthesia signal — the full side effect profile will not be known until the broader Phase 3 TRIUMPH program is completed and reviewed. Long-term safety data from larger and more diverse patient populations over extended treatment periods remain essential.