Cardivo guides

Retatrutide side effects: what the trials show

Updated October 30, 2026 · Wellness education, not medical advice

Key takeaways

The most common side effects were nausea, diarrhea, constipation and vomiting — they rose with the dose.
Dysesthesia (unusual skin sensations such as tingling or burning) affected up to about 12% on higher doses.
At the 12 mg dose, about 11% of people stopped because of side effects in TRIUMPH-1, vs. 4.9% on placebo.
Products sold online as "reta peptides" are unregulated and have been linked to serious harm, including liver failure reports.

Retatrutide side effects list

The clearest picture of retatrutide's side effects comes from Eli Lilly's Phase 3 TRIUMPH-1 trial, which followed 2,339 adults with obesity or overweight for 80 weeks. Lilly reported these rates for the 4 mg, 9 mg and 12 mg doses compared with placebo:

Side effect	4 mg	9 mg	12 mg	Placebo
Nausea	28.6%	38.4%	42.4%	14.8%
Diarrhea	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%
Dysesthesia	5.1%	12.3%	12.5%	0.9%
Urinary tract infection	7.5%	8.8%	8.4%	5.3%
Stopped because of side effects	4.1%	6.9%	11.3%	4.9%

In TRIUMPH-2, which studied people with type 2 diabetes, the pattern was similar but somewhat milder: nausea in 13.7–28.0%, diarrhea in 27.4–33.6%, vomiting in 5.5–15.7%, decreased appetite in up to 17.1%, and dysesthesia in up to 7.3% of people on retatrutide.

Digestive side effects: the most common by far

Like semaglutide and tirzepatide, retatrutide slows the emptying of the stomach and acts on appetite centers in the brain. That is part of how it works, but it also explains why nausea, diarrhea, constipation, vomiting, reflux and burping top the list. In the trials these effects were mostly mild to moderate and tended to appear during the first weeks and after each dose increase, then settle as the body adapted. That is why the trials started at a low dose — 2 mg — and increased it in steps over several months rather than starting at the target dose. People in the trials were generally advised to eat smaller meals, stop eating when they felt full, avoid very fatty or fried food, and drink enough fluids. Severe or persistent vomiting matters because it can lead to dehydration and, in turn, kidney problems; anyone with severe belly pain should seek care, because incretin medicines have been linked with gallbladder problems and, rarely, pancreatitis.

What is dysesthesia?

Dysesthesia means an unpleasant or unusual skin sensation — tingling, burning, prickling, sensitivity to touch or a "sunburn" feeling — without an obvious skin cause. It was noticeably more common with retatrutide than with placebo, especially at 9 and 12 mg, and it is one of the side effects that sets retatrutide apart from older incretin drugs. Lilly reported that most cases were mild to moderate. Researchers are still working out why it happens; rapid weight loss and the glucagon component are among the ideas being studied.

Heart rate and other effects

Incretin medicines usually raise resting heart rate by a few beats per minute. In the Phase 2 retatrutide trial the increase was dose-dependent, peaked around 24 weeks and then declined. Blood pressure, by contrast, tends to fall as weight comes off. Other effects reported with this class include fatigue, hair thinning during rapid weight loss, injection-site reactions, and loss of some muscle along with fat — which is why protein intake and strength training are often recommended. People taking insulin or sulfonylureas alongside these drugs can develop low blood sugar.

Retatrutide side effects in women

Many people search specifically for side effects in women. Published trial reports so far present results for all participants rather than detailed results by sex, but across the incretin class women tend to report nausea and vomiting more often than men. Two practical points matter for women of childbearing age. First, these medicines are not used in pregnancy, and trials require effective contraception; rapid weight loss can also improve fertility, so unplanned pregnancy is a real possibility. Second, tirzepatide is known to reduce how well oral contraceptive pills are absorbed around the start of treatment and after dose increases, and doctors may advise a backup method — the same question will need an answer for retatrutide when it reaches the market. Hair shedding during rapid weight loss is also commonly reported by women and usually recovers.

Long-term side effects

The longest results available run to about two years (the 104-week extension of TRIUMPH-1). We do not yet know about effects over five or ten years, which is why large outcome trials on heart, kidney and other conditions are still running. Questions researchers are watching include the long-term effect of the glucagon component on heart rate and rhythm, bone and muscle health after large weight loss, and what happens when people stop — with all incretin medicines, a good part of the lost weight tends to come back after treatment ends.

Grey-market "reta peptides": the biggest risk today

Retatrutide is not approved anywhere, so it cannot legally be sold as a medicine. Products marketed online as "reta", "reta peptides", "retatrutide peptide" or "research use only" vials are not made or tested by Eli Lilly. Regulators and patient-safety groups warn that such products may not contain the strength they claim, may contain a different ingredient altogether, and may be made in facilities that cannot produce sterile injectables, sometimes with faked certificates of analysis. Dosing mistakes with self-mixed powders have sent people to hospital, and Australian regulators have reported several cases of acute liver failure after black-market retatrutide injections. A 2026 study also found that people using black-market retatrutide lost far less weight than trial participants. If you want retatrutide, the only legitimate route today is a clinical trial — see FDA approval status and how to join a trial.

When to get medical help

Seek urgent care for severe or persistent belly pain (with or without vomiting), signs of dehydration such as very little urine or dizziness on standing, yellowing of the skin or eyes, a racing or irregular heartbeat with dizziness or chest pain, or signs of a serious allergic reaction.

Track what matters while you work on your weight. Whatever medicine or plan you and your doctor choose, the numbers that change first are often your resting heart rate, your blood pressure and your blood sugar. Cardivo measures heart rate and HRV from your fingertip on the iPhone camera in about 30 seconds — no Apple Watch needed — and keeps cuff blood pressure, glucose and mood in one ad-free journal, with weekly trends and a PDF you can bring to your appointments.

This guide is general wellness education based on the public sources below. It is not medical advice, diagnosis, or treatment. Cardivo is not a medical device. If you think you are having a medical emergency, call 911.

Frequently asked questions

What are the most common side effects of retatrutide?

Nausea, diarrhea, constipation and vomiting were the most common in Phase 3 trials, followed by dysesthesia (unusual skin sensations), decreased appetite and urinary tract infections.

How many people stopped retatrutide because of side effects?

In TRIUMPH-1, 4.1%, 6.9% and 11.3% stopped on the 4 mg, 9 mg and 12 mg doses, compared with 4.9% on placebo.

What is dysesthesia on retatrutide?

Dysesthesia is an unusual skin sensation such as tingling, burning or sensitivity to touch. It affected up to about 12% of people on higher retatrutide doses and was mostly mild to moderate.

Are reta peptides sold online safe?

No. Retatrutide is not approved, and products sold online are unregulated. They may have the wrong strength or ingredients and have been linked to serious harm, including reports of liver failure.

Sources

Cardivo is an ad-free iPhone app that reads heart rate and HRV from your fingertip on the camera in about 30 seconds — no Apple Watch needed — and keeps blood pressure, blood sugar, and mood in one journal you can share with your doctor as a PDF. Download on the App Store.