Retatrutide side effects: what the trials reported, and what is still unknown.
Retatrutide is an investigational Eli Lilly medicine that is not approved in Australia or anywhere else. Everything known about its side effects comes from clinical trials: one Phase 2 trial published in 2023 and five Phase 3 trials reported between December 2025 and September 2026. This page sets out what those trials found, what they cannot yet tell us, and the separate, serious risks of unregulated products sold under the retatrutide name.
In Lilly's trials, the most common side effects of retatrutide were gastrointestinal: nausea, diarrhoea, constipation and vomiting. They were mostly mild to moderate and most frequent while the dose was being increased. Abnormal skin sensations (dysaesthesia) and, in Phase 2, a rise in heart rate were also reported. Retatrutide is unapproved, its long-term safety is not established, and unregulated products carry separate, serious risks.
- Unapproved. The TGA states retatrutide "has not been approved by the TGA or any comparable international regulator", so no regulator has reviewed its full safety data. Source: TGA safety alert, 14 September 2026.
- Mostly company-reported. The Phase 2 trial (NEJM, June 2023), TRANSCEND-T2D-1 (The Lancet, June 2026) and TRIUMPH-2 (The Lancet, 29 September 2026) have journal publications. The other Phase 3 side-effect figures on this page are from Lilly topline releases. Sources: NEJM; Lilly, 29 September 2026.
- Heart rate: Phase 2 only. The Phase 3 topline releases reviewed for this page do not give heart-rate figures.
- Harms from unapproved products. Victoria has linked unapproved products labelled Retatrutide to six cases of acute liver injury (alert of 19 June 2026), and the TGA has reported a counterfeit vial linked to a torn oesophagus (14 September 2026). Sources: Victorian Department of Health; TGA.
What are the most common side effects of retatrutide?
Gastrointestinal effects: nausea, diarrhoea, constipation and vomiting, rising with dose. TRIUMPH-1 is the largest trial reported, with 2,339 adults without diabetes treated for 80 weeks. Its rates show the pattern seen across the programme.
| TRIUMPH-1 (80 weeks) | 4 mg | 9 mg | 12 mg | Placebo |
|---|---|---|---|---|
| Nausea | 28.6% | 38.4% | 42.4% | 14.8% |
| Diarrhoea | 25.2% | 34.1% | 32.0% | 13.5% |
| Constipation | 23.8% | 25.9% | 26.1% | 10.9% |
| Vomiting | 10.6% | 22.8% | 25.3% | 4.8% |
| Dysaesthesia (skin sensation) | 5.1% | 12.3% | 12.5% | 0.9% |
| Urinary tract infection | 7.5% | 8.8% | 8.4% | 5.3% |
| Stopped because of adverse events | 4.1% | 6.9% | 11.3% | 4.9% |
Source: Lilly TRIUMPH-1 topline release, 21 May 2026 (company-reported). Placebo rates show how often the same complaints occurred without the medicine.
The Phase 2 paper describes the same profile: gastrointestinal events were the most common, dose-related, mostly mild to moderate, and happened mainly during dose escalation. Starting at a lower dose partly reduced them. In the Phase 3 programme, everyone started on a low dose that was stepped up gradually to the target dose.
How common were side effects in each retatrutide trial?
Rates varied between trials, but every trial reported more gastrointestinal effects and skin-sensation changes on retatrutide than on placebo. The table compares the highest dose, 12 mg, with placebo.
| Trial (12 mg vs placebo) | Nausea | Diarrhoea | Vomiting | Skin sensation | Stopped due to adverse events |
|---|---|---|---|---|---|
| Phase 2, no diabetes, 48 weeks | 45% vs 11% | 15% vs 11% | 19% vs 1% | 13% vs 1%* | 6–16% across retatrutide groups vs 0% |
| TRIUMPH-1, no diabetes, 80 weeks | 42.4% vs 14.8% | 32.0% vs 13.5% | 25.3% vs 4.8% | 12.5% vs 0.9% | 11.3% vs 4.9% |
| TRIUMPH-2, type 2 diabetes, 80 weeks | 28.0% vs 8.0% | 33.6% vs 13.2% | 15.7% vs 4.2% | 7.3% vs 0.7% | 7.7% vs 4.9% |
| TRIUMPH-3, cardiovascular disease, 80 weeks | 22.4% vs 5.8% | 24.4% vs 8.7% | Not given | 6.4% vs 1.3% | 13.5% vs 4.8% |
| TRIUMPH-4, knee osteoarthritis, 68 weeks | 43.2% vs 10.7% | 33.1% vs 13.4% | 20.9% vs 0.0% | 20.9% vs 0.7% | 18.2% vs 4.0% |
| TRANSCEND-T2D-1, type 2 diabetes, 40 weeks | 26.5% vs 3.7% | 22.8% vs 4.5% | 17.6% vs 2.2% | 4.4% vs 0.0% | 5.1% vs 0.0% |
*Phase 2 reported "hyperesthesia or related" events rather than dysaesthesia. Constipation on 12 mg: 16% vs 3% (Phase 2), 16.8% vs 9.4% (TRIUMPH-2), 15.7% vs 7.1% (TRIUMPH-3), 25.0% vs 8.7% (TRIUMPH-4). TRIUMPH-4 also reported decreased appetite in 18.2% on 12 mg vs 9.4% on placebo. Different populations and durations mean these trials should not be ranked against each other.
What is dysaesthesia, the skin-sensation side effect?
Dysaesthesia means abnormal skin sensations, such as tingling, burning or sensitivity to touch. In every retatrutide trial reported so far it was more common on retatrutide than on placebo.
The highest rate so far was in TRIUMPH-4: 8.8% on 9 mg and 20.9% on 12 mg, compared with 0.7% on placebo. Lilly described these events as "generally mild" and said they "rarely led to treatment discontinuation". Across the programme, Lilly says most cases were mild to moderate and resolved during treatment. In the diabetes trials, rates were lower (up to 7.3% in TRIUMPH-2 and 4.4% in TRANSCEND-T2D-1).
The 2023 Phase 2 paper reported a similar signal under a different label, "hyperesthesia or related" events, in 13% on 12 mg compared with 1% on placebo. Why retatrutide causes these sensations has not been explained in the sources reviewed for this page.
Why did people stop retatrutide in the trials?
Mostly because of side effects, and more often at higher doses, though the pattern was not identical in every trial.
In TRIUMPH-1, 11.3% of people on 12 mg stopped because of adverse events, compared with 4.9% on placebo; the 4 mg rate (4.1%) was similar to placebo. TRIUMPH-4 had the highest rate: 12.2% on 9 mg and 18.2% on 12 mg, compared with 4.0% on placebo. Lilly said these rates were "highly correlated with baseline BMI" and included people stopping for "perceived excessive weight loss". Among participants with a BMI of 35 or more, the rates were 8.8% and 12.1%, compared with 4.8%.
Not every trial rose neatly with dose. In TRIUMPH-2, 11.6% stopped on 9 mg but 7.7% on 12 mg, compared with 4.9% on placebo. In the Phase 2 trial, 6% to 16% of the retatrutide groups stopped because of adverse events, and no one on placebo did.
Does retatrutide affect heart rate or heart health?
Heart rate rose in Phase 2, and the only heart-outcome result so far is inconclusive.
Heart rate rose in a dose-dependent way, peaked at 24 weeks and then declined. Cardiac arrhythmia events were reported in 11% of people on 12 mg compared with 3% on placebo, and were mostly mild to moderate.
The Lilly topline releases reviewed for this page do not report Phase 3 heart-rate changes. This page will be updated if they are published.
In 1,949 adults with established cardiovascular disease, the hazard ratio for MACE-5 was 0.82 (95% CI 0.55–1.22) and for MACE-3 was 1.12 (0.64–1.96). Neither was statistically significant, and there were fewer events than expected.
Wide confidence intervals on both sides of 1.0 mean the trial neither shows that retatrutide reduces heart events nor rules out an increase.
Is retatrutide safe? What is not yet known.
Its safety has not been established. The trials describe common, short-to-medium-term side effects; they cannot yet answer several larger questions.
- Long-term use. The main Phase 3 trials ran for 40 to 80 weeks. Longer data so far come from a 104-week extension of TRIUMPH-1 in 532 people who tolerated their dose, which is a selected group.
- Heart outcomes. TRIUMPH-3 was inconclusive on major heart events, and Phase 3 heart-rate data have not been released in the toplines reviewed.
- Rare events. Trials of a few thousand people may not detect side effects that are uncommon.
- Independent review. No regulator has assessed the complete safety data, and most Phase 3 figures are company-reported.
For comparison, the side effects of registered medicines are documented in their Australian product information: see Mounjaro side effects and Ozempic side effects.
What extra risks come with unregulated "Reta" products?
The trial data above describe Lilly's medicine. Products sold online as retatrutide may contain something else entirely, and Australian authorities have linked them to harms that are not part of the trial profile.
Liver injury (Victoria). On 19 June 2026, Victoria's Chief Health Officer reported six cases of acute liver injury since January 2026 in people who had used unapproved products labelled Retatrutide, also sold as "Reta", "R-10" and "R-20". Findings included raised liver enzymes, with possible raised bilirubin and blood-clotting problems. The alert says "additional contaminants may be contributing". The ABC later reported at least seven Victorians seriously unwell with liver injury or failure; that is a media figure, not an official count.
Mislabelled contents (TGA). On 14 September 2026 the TGA reported that a person who used a product sold as retatrutide had uncontrollable vomiting, a torn oesophagus and needed hospital treatment. The vial contained no retatrutide, only undeclared semaglutide at about 8 times the level in registered products. The TGA warns that "any retatrutide product accessed outside of clinical trials could be a counterfeit product".
National picture. A joint TGA and Chief Medical Officer statement (19 June 2026) said TGA reports and state and territory hospitalisation data show liver damage, severe allergic reactions requiring hospitalisation, and inflammation linked to unapproved peptide products, including retatrutide. It did not publish numbers. The retatrutide in Australia guide covers the legal position and enforcement.
When to get urgent help, and how to report a problem.
If you have used a product sold as retatrutide and feel unwell, stop using it and see a doctor. In an emergency, call 000.
- Call 000 for any emergency, including vomiting you cannot stop. In the case the TGA reported, uncontrollable vomiting came before a torn oesophagus.
- Call the Poisons Information Centre on 13 11 26 for advice, as Victoria's alert recommends.
- See a doctor about any symptoms and tell them exactly what you used, including any product name such as Reta, R-10 or R-20. Liver injury in the Victorian cases was identified through blood tests.
- Keep the vial and packaging. In the September case, TGA laboratory testing of the vial is how its real contents were identified.
- Report it. The TGA's September alert followed a serious adverse event report. Your doctor or pharmacist can report a suspected reaction to the TGA, and you can report one yourself: TGA, report an adverse event or safety problem.
Food cannot treat or prevent any of these harms. If a smaller appetite is making meals hard, the foods that are easier on a queasy day guide is general food information, not a substitute for medical care.
What are the most common side effects of retatrutide?
In the trials, gastrointestinal effects: nausea, diarrhoea, constipation and vomiting. In TRIUMPH-1, nausea affected 42.4% of people on 12 mg compared with 14.8% on placebo. These effects were dose-related, mostly mild to moderate, and most frequent while the dose was being increased.
Is retatrutide safe?
That has not been established. Retatrutide is investigational and no regulator has approved it. The trials report its short-term side effects, but long-term safety and its effect on heart events are not yet known. Products sold outside trials carry additional risks because their contents are unknown.
What is dysaesthesia on retatrutide?
Dysaesthesia means abnormal skin sensations, such as tingling, burning or sensitivity to touch. It was reported in up to 20.9% of people on 12 mg in TRIUMPH-4, compared with 0.7% on placebo. Lilly says most cases were mild to moderate and resolved during treatment.
How many people stopped retatrutide because of side effects?
It varied by trial and dose. On 12 mg, 11.3% stopped because of adverse events in TRIUMPH-1, compared with 4.9% on placebo, and 18.2% stopped in TRIUMPH-4, compared with 4.0%. Lilly said the TRIUMPH-4 rate was highly correlated with baseline BMI.
Does retatrutide raise heart rate?
In the Phase 2 trial, heart rate rose in a dose-dependent way, peaked at 24 weeks and then declined. Cardiac arrhythmia events were reported in 11% of people on 12 mg and 3% on placebo, mostly mild to moderate. Phase 3 heart-rate figures were not given in the Lilly topline releases we reviewed.
What are the side effects of Reta peptides bought online?
Nobody can say, because the contents are unknown. In Victoria, unapproved products labelled Retatrutide were linked to six cases of acute liver injury. The TGA tested one vial sold as retatrutide and found only undeclared semaglutide at about 8 times the registered strength, after a user was hospitalised with a torn oesophagus.
Related explainers: Retatrutide in Australia · Retatrutide vs tirzepatide · Mounjaro side effects · Weight-loss medication in Australia
Primary sources checked for this page.
- Jastreboff et al., "Triple-Hormone-Receptor Agonist Retatrutide for Obesity: A Phase 2 Trial", NEJM, published online 26 June 2023 (including Table 3).
- Eli Lilly: TRIUMPH-4 topline, 11 December 2025.
- Eli Lilly: TRIUMPH-1 topline, 21 May 2026.
- Eli Lilly: ADA results including TRANSCEND-T2D-1, 6 June 2026.
- Eli Lilly: TRIUMPH-2 and TRIUMPH-3 topline, 23 July 2026.
- Eli Lilly: TRIUMPH-2 detailed results (EASD and The Lancet), 29 September 2026.
- Victorian Department of Health: toxicity linked to unapproved peptide product labelled Retatrutide, 19 June 2026.
- TGA safety alert: counterfeit retatrutide product, 14 September 2026.
- TGA and Chief Medical Officer: public health risks of unapproved peptide products, 19 June 2026.
- the limbic, citing ABC: liver injury cluster, 26 August 2026 (media report).
Editorial method: trial safety tables were taken from the peer-reviewed Phase 2 paper and from Lilly's own releases, which are company-reported and, unless a journal is named, not yet peer reviewed. Australian harm reports were taken from the TGA and the Victorian Department of Health; media figures are labelled as such. Where a figure has not been published, we say so rather than estimate it. Clinical review by a named Australian health professional is pending. See the Preptide editorial policy.
Important: General information only, not medical advice. Retatrutide is an investigational medicine of Eli Lilly that is not approved in Australia and is not lawfully available outside clinical trials; do not purchase or use products sold under its name. Mounjaro is a registered trademark of Eli Lilly, and Ozempic is a registered trademark of Novo Nordisk. Preptide is independent and is not affiliated with, endorsed by or sponsored by Eli Lilly or Novo Nordisk. Preptide is food, not medication, and does not prescribe, sell, recommend or promote prescription medicines. Talk to your doctor or pharmacist about any medicine. In an emergency call 000. Read our editorial policy and Health Acknowledgement.