Safety Profile
Retatrutide Side Effects: TRIUMPH Trial Data
Phase 2 data provides the most complete characterization of retatrutide's side effect profile. Phase 3 TRIUMPH trial safety data is accumulating. This page summarizes what is known and compares incidence rates to approved GLP-1 medications.
⚠ Investigational drug — not FDA-approved
Retatrutide is currently in Phase 3 clinical trials. It is not available by prescription and not approved for human use outside of clinical trials. The December 2025 TRIUMPH-4 readout has now documented dysesthesia (abnormal skin sensation) in 8.8%–20.9% of participants depending on dose. This and other signals will be evaluated in the FDA NDA review process. This page is educational only.
Side Effect Overview: Retatrutide Phase 2 Data
The Phase 2 trial published in NEJM (Jastreboff et al., 2023) enrolled 338 participants across dose groups (1 mg, 2 mg, 4 mg, 8 mg, 12 mg, and placebo) for 48 weeks. The safety profile was broadly consistent with the GLP-1/GIP class, with the addition of some glucagon receptor-related signals.
GI Side Effects: Incidence by Dose
| Side Effect | Retatrutide (all doses) | Semaglutide 2.4 mg (STEP-1) | Tirzepatide 15 mg (SURMOUNT-1) |
|---|---|---|---|
| Nausea | ~40–55% (dose-dependent) | ~44% | ~31% |
| Vomiting | ~15–25% (dose-dependent) | ~24% | ~11% |
| Diarrhea | ~20–30% | ~30% | ~23% |
| Constipation | ~15–22% | ~24% | ~17% |
| Decreased appetite | ~35–50% | ~29% | ~34% |
| Injection-site reaction | ~5–10% | ~4% | ~7% |
| Fatigue | ~8–15% | ~11% | ~9% |
⚠ Retatrutide percentages are from Phase 2 data only and reflect ranges across dose groups. Phase 3 data will provide more definitive estimates. Semaglutide and tirzepatide figures are from published pivotal trials.
Dysesthesia: A New Phase 3 Safety Signal (TRIUMPH-4, December 2025)
⚠ TRIUMPH-4 Dysesthesia Signal — Phase 3 Data
The December 2025 TRIUMPH-4 readout identified dysesthesia (abnormal skin sensation — tingling, crawling, or prickling) as a new safety signal not seen in Phase 2. This is a documented Phase 3 finding, not a theoretical risk.
Dysesthesia is defined as an abnormal, often unpleasant sensation (tingling, crawling, burning, or prickling of the skin) not caused by a physical stimulus. In TRIUMPH-4 (obesity + knee osteoarthritis, 68 weeks), the rates were:
| Arm | Dysesthesia Rate | Discontinuation Rate |
|---|---|---|
| Retatrutide 12 mg | 20.9% | 18.2% |
| Retatrutide 9 mg | 8.8% | 12.2% |
| Placebo | 0.7% | 4.0% |
⚠ This signal was NOT observed in Phase 2 trials. Lilly stated dysesthesia did not appear to be a primary driver of discontinuation. Full characterization awaits peer-reviewed publication. Source: BioSpace, December 2025.
The mechanism underlying this signal is not yet characterized. Glucagon receptor agonism affecting peripheral nerve signaling is one hypothesis under investigation. The 20.9% rate at 12 mg makes dysesthesia the most frequently observed new signal from TRIUMPH-4 and a key regulatory question the FDA will scrutinize in any future NDA review.
Lipase Elevation: A Retatrutide-Specific Signal
A notable finding in Phase 2 was dose-dependent elevation in serum lipase. Elevated lipase can indicate pancreatic stress and is a potential precursor to pancreatitis, though most elevations were asymptomatic and did not meet diagnostic criteria for pancreatitis.
- Lipase elevations ≥3× upper limit of normal were observed in a higher proportion of participants at 8 mg and 12 mg doses
- No cases of acute pancreatitis were reported in Phase 2 at standard escalation rates
- Patients with personal or family history of pancreatitis, medullary thyroid carcinoma, or MEN 2 are excluded from TRIUMPH trials
- Phase 3 safety monitoring includes regular lipase surveillance at each study visit
This signal is a key regulatory question that the FDA will scrutinize in any future NDA review. Similar signals have been studied (though not confirmed as clinically significant at approved doses) with other GLP-1 agents.
Heart Rate Effects
Like other GLP-1 receptor agonists, retatrutide produced modest increases in resting heart rate in Phase 2. The glucagon receptor component may contribute additional chronotropic (heart rate-increasing) effects. Phase 2 data showed mean heart rate increases of approximately 4–8 bpm at higher doses — comparable to semaglutide and tirzepatide.
TRIUMPH-3 includes patients with cardiovascular risk, which will provide important data on the cardiovascular safety profile of retatrutide at maximum doses.
Discontinuation Rates
In Phase 2, discontinuation due to adverse events was approximately:
- Retatrutide 12 mg: ~7% discontinuation rate due to adverse events
- Semaglutide 2.4 mg (STEP-1): ~7.0% due to adverse events
- Tirzepatide 15 mg (SURMOUNT-1): ~4.3% due to adverse events
Retatrutide's discontinuation rate at Phase 2 was broadly similar to semaglutide, and somewhat higher than tirzepatide's 15 mg data. Phase 3 data will be more definitive.
Thyroid C-Cell Tumor Warning
Like all GLP-1 receptor agonists, retatrutide is expected to carry a class boxed warning regarding thyroid C-cell tumors based on rodent studies. The clinical relevance in humans remains uncertain. Patients with personal or family history of medullary thyroid carcinoma (MTC) or Multiple Endocrine Neoplasia type 2 (MEN2) are excluded from trials.
What Phase 3 Will Add
Phase 3 TRIUMPH trials will provide:
- Larger sample sizes (thousands vs hundreds in Phase 2) for rare adverse event detection
- Longer follow-up periods for chronic safety signals
- Cardiovascular safety data (TRIUMPH-3 includes patients with CV risk)
- Definitive pancreatitis incidence data
- Pediatric and older adult subgroup data (in relevant trials)
The FDA will require full Phase 3 safety data before any NDA approval decision.
Sources
- BioSpace — TRIUMPH-4 readout including dysesthesia data (December 2025): Lilly's retatrutide scores triple trial triumph
- Jastreboff AM, et al. "Triple–Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial." NEJM 2023. https://www.nejm.org/doi/full/10.1056/NEJMoa2301972
- Wilding JPH, et al. "Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP-1)." NEJM 2021. https://www.nejm.org/doi/full/10.1056/NEJMoa2032183
- Jastreboff AM, et al. "Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1)." NEJM 2022. https://www.nejm.org/doi/full/10.1056/NEJMoa2206038
- ClinicalTrials.gov TRIUMPH program. https://clinicaltrials.gov/search?term=retatrutide
Compare Side Effects of Approved Medications
Frequently Asked Questions
Are retatrutide's side effects worse than semaglutide or tirzepatide?
The gastrointestinal side effect profile (nausea, vomiting, diarrhea) is similar in incidence to other GLP-1 medications. Retatrutide adds glucagon-driven effects not present in pure GLP-1 agonists: a dose-dependent increase in resting heart rate (mean +5–8 bpm) and modest increases in fasting glucose at higher doses.
Is the heart rate increase from retatrutide dangerous?
In Phase 2 trials, the mean heart rate increase was modest and was not associated with significant adverse cardiovascular events in the study population. However, the long-term cardiovascular safety profile is still being established. The FDA will require careful evaluation; the drug may carry a label restricting use in certain cardiac populations.
Does retatrutide cause hypoglycemia?
In non-diabetic patients, no — GLP-1 agonism is glucose-dependent. In patients taking insulin or sulfonylureas concurrently, dose reduction of those medications is usually required to prevent hypoglycemia, similar to standard GLP-1 medication protocols.
What is the discontinuation rate due to side effects in retatrutide trials?
Phase 2 trials reported discontinuation rates due to adverse events in the 8–11% range across the higher dose arms — comparable to tirzepatide and semaglutide in their respective pivotal trials.
Are there long-term safety concerns specific to triple agonism?
Glucagon receptor activation increases hepatic glucose output and may affect bile acid metabolism. Long-term effects on liver function, body composition (particularly lean mass), and cardiovascular markers will need to be characterized in Phase 3 and post-marketing surveillance.