Retatrutide side effects are back in the headlines after Eli Lilly reported new phase 3 results on September 29, 2026. Retatrutide is an investigational once-weekly injection that has not been approved by the FDA, so everything below comes from clinical trials, not from everyday prescribing. This article explains what the trials report about nausea, diarrhea, unusual skin sensations and stopping treatment, and which blood tests doctors commonly use to follow people taking this kind of medicine.
The aim is to help you read the numbers calmly. A trial result describes groups of volunteers under close supervision. It cannot tell you what will happen to you.
What retatrutide is and where it stands
Retatrutide activates three hormone receptors at once: GLP-1, GIP and glucagon. Semaglutide acts on one of these and tirzepatide on two, which is why retatrutide is often called a triple agonist. In the TRIUMPH program, adults with obesity or overweight, with or without type 2 diabetes, received a weekly injection or a placebo for about 80 weeks.
Lilly’s latest release points to an application to the FDA being planned for early 2027. Until a regulator approves it, retatrutide is not available by prescription.
Retatrutide side effects reported in the phase 3 trials
Digestive effects come first
In TRIUMPH-1, which enrolled 2,339 adults without diabetes, stomach and bowel symptoms were the most common problems at the 12 mg dose. They are summarized below against placebo, as reported by the trial publicity coverage of Lilly’s results.
| Side effect (TRIUMPH-1, 12 mg) | Retatrutide | Placebo |
|---|---|---|
| Nausée | 42.4% | 14.8% |
| Diarrhea | 32.0% | 13.5% |
| Constipation | 26.1% | 10.9% |
| Vomiting | 25.3% | 4.8% |
These events were described as mild to moderate. An earlier phase 2 trial published in the New England Journal of Medicine found that they were dose related and partly softened by a lower starting dose.
Dysesthesia: a newer signal
Dysesthesia means an abnormal or uncomfortable skin sensation, such as tingling, burning or heightened touch. It was not reported in the earlier phase 2 work. In TRIUMPH-4, a trial in people with obesity and knee osteoarthritis, it affected 8.8% of people on 9 mg and 20.9% on 12 mg, versus 0.7% on placebo, according to BioSpace. In TRIUMPH-1 it was described as mild. Lilly reported that these events did not lead people to stop treatment.
Stopping treatment
In TRIUMPH-1, discontinuation because of side effects was 4.1% at 4 mg, 6.9% at 9 mg and 11.3% at 12 mg, compared with 4.9% on placebo. In TRIUMPH-4 the figures were higher, at 12.2% and 18.2% for 9 mg and 12 mg versus 4% on placebo, which Lilly partly attributed to participants feeling their weight loss was excessive.
What the new TRIUMPH-2 results add
TRIUMPH-2 followed 1,152 adults with type 2 diabetes and obesity or overweight for 80 weeks. At 12 mg, average weight loss was 20.8% against 4.0% on placebo, and A1C fell by about 1.5 to 1.6 points at the two higher doses against 0.2 on placebo. Digestive effects were again the main issue, including diarrhea in 33.6% and nausea in 28.0% at 12 mg, against 13.2% and 8.0% on placebo. Discontinuation for side effects was 7.7% versus 4.9%.
Two cautions apply. These figures come from a company press release, and full peer-reviewed publication is still to come. Results in people with diabetes also tend to differ from results in people without it.
Blood tests doctors may use to follow GLP-1-type treatment
No blood test predicts how retatrutide will suit you. The table below lists tests that doctors commonly use for people on GLP-1-class medicines or with the conditions involved. It reflects general practice and common sense, not a monitoring schedule set by the retatrutide trials, so your own doctor decides what you need.
| Test | Why it may be ordered | What to keep in mind |
|---|---|---|
| HbA1c et glycémie à jeun | Track blood sugar over weeks to months | Trials showed clear A1C drops in type 2 diabetes. Other diabetes medicines may need review. |
| Bilan lipidique | See how weight loss shifts cholesterol and triglycerides | Trials reported improvements in non-HDL cholesterol and triglycerides. |
| ALT et AST | Check liver enzymes, often raised in fatty liver disease | The public summaries we reviewed do not detail enzyme changes. |
| Créatinine, DFGe et électrolytes | Look for dehydration after vomiting or diarrhea | Worth checking if digestive symptoms persist. |
| Lipase | Investigate severe, lasting upper-abdominal pain | Not a routine screening test, and one result alone does not diagnose pancreatitis. |
To see how sugar control is measured, read our guide to the HbA1c test. Liver enzymes are explained in our guide to ALT levels and their causes, which you can read before an appointment. Many labs report an eGFR value for kidney filtration, and you can read our explainer on amylase and lipase for the pancreas. For cholesterol beyond the basics, review the advanced lipid panel.
Dernières avancées scientifiques
The first large phase 2 trial, published in the New England Journal of Medicine in 2023, showed that retatrutide produced substantial weight loss in adults with obesity and that digestive effects depended on the dose. It also found that heart rate rose with dose before easing later on. What this changes for you: a slower start may be easier to tolerate, and your doctor may watch your pulse as well as your blood tests.
A 2026 systematic review and meta-analysis pooled the early randomized trials. It found that people stopped treatment because of side effects more often than on placebo, while serious adverse events were uncommon. The authors stressed that the evidence is limited because few trials were available and phase 3 data were not yet fully published. What this changes for you: the promise is real but the long-term picture is unfinished, which is why a regulator and your doctor should guide any decision.
Why unapproved retatrutide is a different risk
Because demand is high, products sold online as retatrutide or as a research peptide are widespread. The FDA warns that unapproved versions of GLP-1 drugs are not reviewed for safety, effectiveness or quality, and it had received more than 1,700 adverse event reports about compounded GLP-1 products as of May 2026. Trial results cannot be applied to a vial of unknown content and strength. If you already use such a product and feel unwell, tell a clinician what you took.
Quand consulter votre médecin
Contact a doctor promptly if you have vomiting or diarrhea that will not stop, signs of dehydration such as dizziness or very dark urine, or severe pain in the upper abdomen that spreads to the back. Seek urgent care for a swollen face, trouble breathing, or chest pain. New burning, tingling or pain in the skin also deserves a conversation, especially if it lasts.
Glossaire
- Triple agonist: a drug that switches on three hormone receptors, here GLP-1, GIP and glucagon.
- Dysesthesia: an unpleasant or abnormal sensation of touch, such as burning or tingling.
- HbA1c: a blood test that reflects average blood sugar over roughly three months.
- Placebo: an inactive injection used for comparison in a trial.
- Phase 3 trial: a large late-stage study that tests whether a treatment works and is safe enough.
Questions fréquemment posées
Is retatrutide FDA approved?
No. It is investigational, and Lilly has said it plans to apply to the FDA. Until a decision is made, it cannot be prescribed in the United States.
What are the most common retatrutide side effects?
Nausea, diarrhea, constipation and vomiting were the most frequent in the phase 3 trials, usually mild to moderate. Decreased appetite also appeared in the diabetes trial.
What is the tingling or burning side effect?
It is called dysesthesia. It was more common at 12 mg than at 9 mg in TRIUMPH-4 and was described as mild in TRIUMPH-1.
Which blood tests are useful during treatment with this kind of drug?
Your doctor may check HbA1c, lipids, liver enzymes and kidney function, and may order lipase only if severe abdominal pain appears. The right set depends on your health history.
Is research-grade retatrutide sold online safe?
The FDA states that unapproved GLP-1 products are not checked for safety, effectiveness or quality. Trial data do not apply to them.
Sources
- FDA: FDA’s Concerns with Unapproved GLP-1 Drugs Used for Weight Loss
- NIDDK (NIH): Prescription Medications to Treat Overweight and Obesity
- Eli Lilly: retatrutide TRIUMPH-2 results, September 29, 2026
- AJMC: Retatrutide in the phase 3 TRIUMPH-1 trial
- Pharmacy Times: Retatrutide in the pivotal phase 3 TRIUMPH-1 trial
- BioSpace: Retatrutide TRIUMPH-4 results and the dysesthesia safety signal
- Jastreboff et al., New England Journal of Medicine, 2023: triple-hormone-receptor agonist retatrutide for obesity, phase 2 trial
- Efficacy and safety of high-dose retatrutide in adults with obesity and type 2 diabetes: systematic review and Bayesian meta-analysis (SN Comprehensive Clinical Medicine, 2026)
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