Retatrutide Side Effects: What the Trials Have Reported So Far

In clinical trials, retatrutide‘s most common side effects were digestive: nausea, diarrhea, constipation, and vomiting. They showed up more often at higher doses and mostly early on. Trials also reported a skin tingling or burning feeling (called dysesthesia), a rise in heart rate, and more people quitting because of side effects than on a placebo, a dummy shot with no drug in it (11.3% versus 4.9% at the highest dose in the main obesity trial, with a smaller gap in some other trials). [2]

Retatrutide is investigational and not FDA-approved for any use, and the FDA says it cannot be used in compounding under federal law, as of October 2026. [1] Every number on this page comes from the real drug, made by Eli Lilly and given under medical supervision. None of it tells you what a vial sold online as “Reta” will do.

What are the most common side effects of retatrutide?

Stomach and gut problems were by far the most common side effects in every trial. In TRIUMPH-1, the main phase 3 obesity trial (the large, final stage of testing before a company asks the FDA for approval), about 4 in 10 people in the highest-dose group had nausea. [2]

Here’s what Lilly reported for the 80-week trial. Each column is a group of trial participants who got one dose of the drug, compared with people who got a placebo. AJMC reported the same Lilly figures for the highest dose. [2] [11]

TRIUMPH-1 (people with obesity or overweight, without diabetes) — company-reported topline, May 21, 2026

[NEEDS VERIFICATION: confirm these against the TRIUMPH-1 paper in NEJM, Sept 29, 2026 (doi 10.1056/NEJMoa2604169) and its supplementary tables; the paper could not be fetched for this draft or the steelman pass. Lilly’s May 21 release and AJMC match each other. Use the published figures if they differ.]

The two other completed obesity trials showed the same pattern, at somewhat lower rates. TRIUMPH-2 enrolled people with obesity and type 2 diabetes. TRIUMPH-3 enrolled people with obesity and existing heart disease. [4]

TRIUMPH-2 and TRIUMPH-3 — company-reported, July 23, 2026

Lilly describes these events as generally mild to moderate, and says most resolved while people kept taking the drug. [4] Keep in mind that Lilly makes retatrutide and has a commercial stake in how its side effects are described. Full results for TRIUMPH-3 haven’t been published in a journal yet.

Gut side effects tended to cluster in the early weeks, when trial doses were still being stepped up. The phase 2 trial paper in the New England Journal of Medicine reports that these events happened mainly during that stepping-up period and were mostly mild to moderate. [7] It’s the same pattern seen across this family of drugs: tirzepatide’s FDA label says most of its nausea, vomiting, and diarrhea happened during dose increases and eased over time. [13] It’s a description of what happened in trials, not a schedule to follow.

How side effects change with dose

In general, higher doses meant more side effects and more people quitting. But the link isn’t perfectly neat, and retatrutide’s dropout numbers vary a lot from trial to trial.

The clearest measure is how many people stopped the drug because of side effects. Here’s what Lilly reported for each trial. [2] [3] [4]

Stopped treatment because of side effects (company-reported)

Two things stand out. In TRIUMPH-1, slightly fewer people quit in the lowest-dose group than on placebo. [2] And in TRIUMPH-2, the highest-dose group’s dropout rate (7.7%) was lower than the middle dose (11.6%) and not far above placebo (4.9%). [3] So it isn’t simply “more drug, more dropouts.”

Most people who had gut side effects didn’t quit over them. In TRIUMPH-1, gut problems led 2.2% to 4.6% of people on retatrutide to stop, versus 1.2% on placebo, according to Lilly’s own poster at the American Diabetes Association’s 2026 meeting. [16]

An independent analysis pooled three earlier, smaller trials with 878 people in total. Its authors reported no funding or conflicts. It found the lowest dose didn’t significantly raise overall side effects or dropouts compared with placebo, while the higher doses did. [17] The authors called retatrutide’s safety profile “generally favorable,” but that analysis came before the large phase 3 trials and noted that long-term safety data were lacking. [17]

That matters because the 11.3% figure gets quoted a lot on its own. It’s real, but it’s one of the higher numbers from one trial, not the whole picture: TRIUMPH-3’s top dose was higher still (13.5%), and TRIUMPH-2’s was lower (7.7%). [3] [4] Comparing dropout rates across different drugs’ trials is rough, because the trials enrolled different people and raised doses on different schedules. Our retatrutide page covers how it compares with semaglutide and tirzepatide trials.

Skin tingling and burning (dysesthesia)

Retatrutide caused an unusual skin sensation in a meaningful share of people: tingling, burning, or skin that feels sore to the touch. Doctors call this dysesthesia, which means an abnormal, unpleasant feeling on the skin without an obvious cause. Lilly defines it in these trials as “a skin burning sensation and related” events. [5]

Lilly’s medical information page lists the rates for all four phase 3 trials that have reported. Placebo rates were 1% in every trial. [5]

Dysesthesia by trial and dose (Lilly Medical, last reviewed Aug 18, 2026)

⚠ NEEDS MEDICAL REVIEW

Lilly says these events were mostly mild to moderate. [5] In the earlier phase 2 trial, skin sensitivity events affected 7% of people on retatrutide versus 1% on placebo. The trial paper reports they were mild to moderate and none led anyone to quit. [7]

That’s reassuring on severity, but it’s not the whole story. At the top dose, rates reached 13% in TRIUMPH-1 and 21% in TRIUMPH-4, many times higher than placebo. [5] Lilly’s page doesn’t say how many people in the phase 3 trials stopped the drug because of it. Lilly’s TRIUMPH-1 release says only that most people with these events kept taking retatrutide. [2]

This is one place retatrutide genuinely stands out. Dysesthesia and skin burning don’t appear among the side effects in the FDA label for tirzepatide (Zepbound), a closely related approved drug. [13] Why retatrutide causes it isn’t explained in the sources we reviewed. ⚠ NEEDS MEDICAL REVIEW

Heart rate and heart safety

Retatrutide raised heart rate, and the rise was bigger at higher doses. In the phase 2 trial, the increase peaked around week 24 and then declined through week 48, according to the trial paper. [7]

How big was it? An independent review in Expert Opinion on Investigational Drugs (2023) reported increases of up to about 6.7 beats per minute in phase 2. [9] The phase 2 NEJM paper’s main text confirms the dose-dependent rise and week-24 peak but puts bpm figures in supplementary Table S6, which we couldn’t fetch. Confirm 6.7 bpm there.] The same reviewer said that rise “may be detrimental and offset some of the benefits of weight loss.” [9]

Some context helps here. A faster resting heart rate is a known effect of this drug family. The FDA labels for tirzepatide (Zepbound) and semaglutide (Wegovy) list average increases of 1 to 3 and 1 to 4 beats per minute. [13] [19] That rise didn’t stop semaglutide from lowering heart risk: in its large heart-outcomes trial in people with obesity and heart disease, heart attack, stroke, or heart-related death was 20% less common on the drug than on placebo. [19]

That’s evidence about semaglutide, though, not retatrutide. Retatrutide’s reported rise (up to about 6.7 beats per minute) is larger than those labeled averages. [9] In the phase 2 trial, irregular heartbeats (arrhythmias) were reported in about 6% of people on retatrutide versus 3% on placebo, and all but one were mild to moderate. The one severe case, a heart-rhythm change called QT prolongation, happened in someone also taking the anti-nausea drug ondansetron. [7] Lilly’s phase 3 releases don’t report heart-rate figures or arrhythmia rates. [2] [3] [4] A clinical pharmacist writing in Pharmacy Times says people who already have fast-heart-rhythm problems warrant closer monitoring. [6]

What about serious heart events? TRIUMPH-3 enrolled people who already had heart disease. Major heart events (death from any cause, heart attack, stroke, heart failure events, or procedures to reopen blocked heart arteries) occurred 44 times across the retatrutide groups versus 52 times on placebo. [4] Using the narrower classic definition (heart-related death, heart attack, or stroke), it was 27 versus 23. [4]

Read that carefully. These are Lilly’s own numbers, and Lilly makes the drug. The statistical range around both results was wide, meaning the true effect could be meaningfully better or worse than it looks: for the broader measure, anywhere from about 45% fewer heart events to about 22% more. [4] Lilly also said heart events happened less often than expected in both groups, which makes the comparison less certain. [4] The fair reading is “no clear increase in heart events,” not “proven heart-safe.” The full paper isn’t out yet.

Does retatrutide cause hair loss?

We don’t know yet. Lilly’s published trial summaries don’t report a hair-loss rate for retatrutide. [2] [3] [4]

It’s a fair question, though. Hair loss is a listed side effect of tirzepatide (Zepbound), a drug from the same family. In tirzepatide’s trials, 4% to 5% of people on the drug reported hair loss versus 1% on placebo, and the FDA label ties it to weight reduction. [13] Losing a lot of weight can trigger temporary hair shedding on its own.

But that’s evidence about tirzepatide, not retatrutide. Until a retatrutide paper reports a number, any hair-loss rate you see quoted for it isn’t coming from trial data we can find.

Side effects reported in women

The one sex-related signal in the trial reports so far is urinary tract infections (UTIs), meaning bladder or urinary infections. In TRIUMPH-1, UTIs were reported in 7.5% to 8.8% of people on retatrutide versus 5.3% on placebo. [2]

Lilly’s poster at the American Diabetes Association’s 2026 meeting, which reported slightly different figures (6.8% to 8.1% versus 4.8%), said 92% of these UTIs were in female participants. [16] Lilly describes the UTIs as generally mild to moderate, and says most resolved during treatment. [2] The gap was smaller in TRIUMPH-2 (8.0% at the top dose versus 6.6% on placebo) and TRIUMPH-3 (7.0% versus 5.3%). [4]

Lilly’s summaries don’t break out other side effects by sex. For tirzepatide, a related drug, the FDA label notes hair loss was reported more often in women than men. [13] Whether that holds for retatrutide hasn’t been reported.

Other side effects to know about

Low blood pressure. In TRIUMPH-1, some people had symptoms of low blood pressure (hypotension), such as dizziness or lightheadedness. The researchers found this was dose-dependent and more common in people already taking blood-pressure medication, as reported by HCPLive and in Lilly’s ADA 2026 poster. [10] [16] In the phase 2 diabetes trial, blood pressure dropped enough that many participants stopped at least one blood-pressure drug. [6] If you take blood-pressure medication, that’s relevant.

Decreased appetite. Lilly lists it as a common side effect in TRIUMPH-2 and TRIUMPH-3, reaching 17.1% at the top dose versus 4.5% on placebo in TRIUMPH-2. [4] For a weight-loss drug, that’s partly the point. But it’s still tracked as a side effect.

Tiredness. People ask about this a lot. Lilly’s trial summaries don’t report a rate for fatigue or tiredness. [2] [3] [4] ⚠ NEEDS MEDICAL REVIEW

Gallbladder problems, pancreatitis, and serious side effects. Gallbladder problems and pancreatitis (a painful inflammation of the pancreas) are watched closely with drugs in this family. Lilly’s topline releases don’t report rates for either. [2] [3] [4] Lilly’s ADA 2026 poster did report serious side effects overall for TRIUMPH-1: 7.7% to 10.5% of people on retatrutide versus 5.5% on placebo (company-reported). [16]

The earlier, smaller trials point both ways. An independent pooled analysis of four phase 1 and 2 trials (the authors reported no funding or conflicts) found no significant difference in serious side effects. [18] The same analysis found a combined group of closely watched events, including pancreatitis and allergic reactions, was about 3 times as common on retatrutide, though it couldn’t separate the two. [18] [NEEDS VERIFICATION: gallbladder and pancreatitis rates in the TRIUMPH-1 NEJM paper/supplement.]

Does retatrutide cause cancer?

No trial has reported a cancer signal for retatrutide in the summaries published so far. [2] [3] [4] That’s not the same as proof it’s cancer-free, because rare problems can take longer and more people to show up.

Here’s where the question comes from. Approved drugs in the same family carry the FDA’s strongest warning, a boxed warning, about thyroid tumors. For tirzepatide, the label says that in rats, the drug causes thyroid C-cell tumors (a type of thyroid tumor), with more tumors at higher doses and longer treatment. [13] The label says whether that applies to people hasn’t been determined.

Retatrutide has no FDA label yet, so no FDA-reviewed warning of any kind exists for it. That gap is a reason for caution, not evidence of a cancer risk.

How long do side effects last, and what happens after stopping?

In the trials, most side effects were described as temporary. Lilly says the majority resolved while people kept taking the drug, and gut side effects were concentrated in the early, dose-increasing weeks. [2] [4] [7]

The longest follow-up so far is 104 weeks, about 2 years. That comes from an extension of TRIUMPH-1 in 532 participants. [2] Nothing longer has been reported.

There’s no published data on what happens to side effects after someone stops retatrutide. TRIUMPH-2 included only a 4-week follow-up after treatment ended. [3] We don’t know whether any effects linger, or what happens to weight, appetite, or heart rate in the months afterward. For how long people have taken it in trials, see how long you can take retatrutide.

Trial side effects vs. the risks of “Reta” bought online

Every side-effect rate above comes from Lilly-made retatrutide given in a supervised trial. Nothing in those trials tells you what an unregulated vial will do.

Here’s who actually sells it right now. There’s no prescription version at any pharmacy, and the FDA says compounding pharmacies can’t legally make it. [1] The only ways to get the authentic drug are a clinical trial or Lilly’s narrow early-access program, which your doctor has to request and which is limited to people with severe obesity and serious complications. [12] Everything else is sold outside those channels. That includes “research use only” websites selling straight to consumers and some telehealth or med spa sellers, both of which the FDA has warned. [1]

A “research use only” label means no FDA oversight of how it’s made. No one has verified what’s in the vial, how strong it is, or whether it’s sterile, and there’s no medical supervision. A clinician being involved through a telehealth site doesn’t change that: the product still isn’t approved or verified. [1] Some research-use vendors publish independent third-party lab results, which is more than many do. But a test of one sample isn’t the FDA manufacturing oversight approved drugs get, and it doesn’t make the product approved or legal for human use. [1] Lilly, which has a commercial interest in steering people away from these products, says it has found products sold as retatrutide with too much, too little, or the wrong active ingredient. [14]

In October 2026, CBS News reported two deaths in the FDA’s adverse-event database after people used products sold as retatrutide from unofficial sources. [14] CBS said it couldn’t verify the reports. What was in those products was never confirmed, and other factors were present: kava use in one case, severe malnutrition and dehydration in the other. The FDA itself says a report in this database doesn’t mean the drug caused the event. [8] We cover the details in our post on FAERS reports and poison-center cases. ⚠ NEEDS MEDICAL REVIEW

These reports aren’t a retatrutide side effect. They’re a reminder of the separate risk of unregulated products. Track where retatrutide stands with the FDA on our regulatory tracker.

Known vs. not yet known

What the trials have shown:

• The most common side effects are digestive: nausea, diarrhea, constipation, and vomiting. [2]
• Side effects generally increase with dose, though not perfectly. [2] [3]
• Between 3.8% and 13.5% of people stopped because of side effects, depending on trial and dose, versus about 5% on placebo. [2] [3] [4]
• Skin tingling or burning (dysesthesia) affected 4% to 21%, depending on trial and dose, versus 1% on placebo. [5]
• Heart rate rises at higher doses. In phase 2, the rise peaked around week 24 and then declined. [7]
• Serious side effects were somewhat more common on retatrutide than placebo in TRIUMPH-1 (company-reported). [16]
• In people with heart disease, there was no clear increase in major heart events (company-reported). [4]

What isn’t known yet:

• Safety beyond about 2 years. [2]
• What happens after stopping.
• Rates of rare side effects, which need larger and longer studies.
• How the drug affects people who wouldn’t have qualified for the trials.
• Any FDA-reviewed warnings, because there’s no FDA label yet.

Want the bigger picture? Read what retatrutide is and where it stands with the FDA, browse our GLP-1 and weight-loss peptides, or see our tirzepatide page. For a side-by-side look, see [LINK: Retatrutide vs. Tirzepatide — add when live Oct 19]. For an overall verdict on safety, see is retatrutide safe?

Sources

1. U.S. Food and Drug Administration. “FDA’s Concerns with Unapproved GLP-1 Drugs Used for Weight Loss.” Content current as of 10/01/2026. https://www.fda.gov/drugs/drug-alerts-and-statements/fdas-concerns-unapproved-glp-1-drugs-used-weight-loss
2. Eli Lilly and Company. “Lilly’s triple agonist, retatrutide, delivered powerful weight loss in pivotal phase 3 obesity trial” (TRIUMPH-1 topline, company-reported). May 21, 2026, via BioSpace. https://www.biospace.com/press-releases/lillys-triple-agonist-retatrutide-delivered-powerful-weight-loss-in-pivotal-phase-3-obesity-trial
3. Eli Lilly and Company. TRIUMPH-2 results release (company-reported; published in The Lancet). Sept 29, 2026. https://investor.lilly.com/node/54981
4. Eli Lilly and Company. “Lilly’s triple agonist, retatrutide, successful in two additional phase 3 trials” (TRIUMPH-2 and TRIUMPH-3, company-reported). July 23, 2026. https://investor.lilly.com/news-releases/news-release-details/lillys-triple-agonist-retatrutide-successful-two-additional
5. Lilly Medical Information. “Was dysesthesia reported as an adverse event in the retatrutide TRIUMPH trials in patients with overweight or obesity?” Last reviewed Aug 18, 2026. https://medical.lilly.com/us/products/answers/was-dysesthesia-reported-as-an-adverse-event-in-the-retatrutide-triumph-trials-in-patients-with-overweight-or-obesity-387925
6. Goldman J, Halpern L. “What Pharmacists Need to Know About Retatrutide’s TRIUMPH-1 Phase 3 Results.” Pharmacy Times (clinician commentary). June 3, 2026. https://www.pharmacytimes.com/view/what-pharmacists-need-to-know-about-retatrutide-s-triumph-1-phase-3-results
7. Jastreboff AM, et al. “Triple-Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial.” New England Journal of Medicine. 2023. https://www.nejm.org/doi/full/10.1056/NEJMoa2301972
8. U.S. Food and Drug Administration. “FDA Adverse Event Reporting System (FAERS) Public Dashboard.” https://www.fda.gov/drugs/questions-and-answers-fdas-adverse-event-reporting-system-faers/fda-adverse-event-reporting-system-faers-public-dashboard
9. Expert Opinion on Investigational Drugs. Review of retatrutide (independent review). 2023. https://www.informahealthcare.com/doi/abs/10.1080/13543784.2023.2283020
10. HCPLive. “Retatrutide Improves Weight Loss, Knee Osteoarthritis Pain, Sleep Apnea Versus Placebo” (on the TRIUMPH-1 NEJM publication). Sept 29, 2026. https://www.hcplive.com/view/retatrutide-improves-weight-loss-knee-osteoarthritis-pain-sleep-apnea-versus-placebo
11. Grossi G. “Retatrutide Achieves Up to 30.3% Average Weight Loss in Phase 3 TRIUMPH-1 Trial.” AJMC, reporting Lilly’s TRIUMPH-1 topline data. May 21, 2026. https://www.ajmc.com/view/retatrutide-achieves-up-to-30-3-average-weight-loss-in-phase-3-triumph-1-trial
12. Lilly Medical Information. “Is there an expanded access program for retatrutide?” https://medical.lilly.com/us/products/answers/is-there-an-expanded-access-program-for-retatrutide-416668
13. U.S. Food and Drug Administration. Zepbound (tirzepatide) prescribing information. https://www.accessdata.fda.gov/drugsatfda_docs/label/2024/217806s003lbl.pdf
14. CBS News. “Deaths reported to FDA after use of bootleg ‘Reta’ weight-loss drug.” Oct 13, 2026. https://www.cbsnews.com/news/deaths-reported-fda-bootleg-reta-weight-loss-drug/
15. ClinicalTrials.gov. TRIUMPH-1 (NCT05929066) and TRIUMPH-2 (NCT05929079). https://clinicaltrials.gov/study/NCT05929066 · https://clinicaltrials.gov/study/NCT05929079
16. Eli Lilly and Company. “The First Phase 3 Obesity Study of Retatrutide, a GIP, GLP-1, and Glucagon Receptor Agonist, in People with Obesity” (TRIUMPH-1 poster, ADA 2026; company-reported). https://lilly.com/hcp/congresses/ada-2026/dv039292-triumph-1
17. Abouelmagd AA, et al. “Efficacy and safety of retatrutide, a novel GLP-1, GIP, and glucagon receptor agonist for obesity treatment: a systematic review and meta-analysis of randomized controlled trials.” Proceedings (Baylor University Medical Center). 2025;38(3):291–303. Authors report no funding or conflicts. https://pmc.ncbi.nlm.nih.gov/articles/PMC12026077/
18. Maharshi V, et al. “Navigating retatrutide safety: comprehensive insights from systematic review and meta-analysis.” Journal of Public Health and Development. 2026;24(1):318–338. Authors report no funding or conflicts. https://he01.tci-thaijo.org/index.php/AIHD-MU/article/view/277028
19. U.S. Food and Drug Administration. Wegovy (semaglutide) prescribing information (revised 03/2024). https://www.accessdata.fda.gov/drugsatfda_docs/label/2024/215256s011lbl.pdf

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