Retatrutide side effects 2026: what phase 3 trials show

Mostly stomach and gut problems: nausea, diarrhea, constipation and vomiting, and they get more common as the dose goes up. That is the clear pattern across the retatrutide side effects reported in phase 3, and it now rests on strong human evidence from large placebo controlled trials.

The timing is why we wrote this now. On September 29, 2026, the full TRIUMPH-1 results came out in the New England Journal of Medicine, and the TRIUMPH-2 trial in people with type 2 diabetes came out in The Lancet the same day. Within a day, retatrutide posts were among the most liked peptide content on X, and side effect questions filled Google's autocomplete.

TRIUMPH-1 enrolled 2,339 adults with obesity and no diabetes for 80 weeks. Lilly's topline release listed these rates:

Look at the placebo column first. About one in seven people on a dummy shot also reported nausea, so part of what gets blamed on the drug is background noise. The gap between the columns is the drug's real contribution.

Sources:s1, s3

Dysesthesia is an odd or unpleasant skin sensation, such as tingling, burning, prickling, or skin that feels sore to the touch with nothing visibly wrong. Of all the retatrutide side effects on the list, it is the one most people have never heard of, and the trial data are strong human evidence that it happens.

In TRIUMPH-1 it showed up in 12.5 percent of people on the 12 mg dose, against 0.9 percent on placebo. In TRIUMPH-2, which enrolled people with obesity and type 2 diabetes, the rates were lower: 4.5, 5.6 and 7.3 percent across the three doses, against 0.7 percent on placebo.

Lilly described the TRIUMPH-1 cases as generally mild to moderate and said most resolved while people kept taking the drug. That is the company's own summary, and the abstracts we read do not say how long episodes lasted. The published summaries also do not explain why the drug causes it, so treat any confident mechanism you see on Reddit as a guess.

New numbness, weakness, or burning that keeps spreading is a reason to see a clinician, whatever you are taking.

Sources:s2, s3, s4

It can raise resting heart rate, at least for a while. In the 2023 phase 2 trial in the New England Journal of Medicine, heart rate rose with dose, peaked around week 24, and then drifted back down. That is limited human evidence from 338 people, and the phase 3 summaries we read do not report heart rate numbers yet.

Blood pressure can also drop too far in some people. TRIUMPH-2 reported low blood pressure (hypotension) in 1, 5 and 6 percent of people on the 4, 9 and 12 mg doses, against under 1 percent on placebo. If you already take blood pressure medicine, that is worth raising with whoever prescribes it.

The bigger heart question is still open. In TRIUMPH-3, a trial in people with obesity and existing cardiovascular disease, there were 27 major heart events (heart attack, stroke, or cardiovascular death) on retatrutide versus 23 on placebo, BioPharma Dive reported in July. Lilly said these happened less often than expected, and analysts cautioned that the counts are too small to draw firm conclusions.

The trial built to settle this, TRIUMPH-Outcomes (NCT06383390), plans to enroll about 10,000 adults and is not expected to finish until February 2029. Until then, the long term picture for the heart and kidneys is unknown.

Sources:s2, s6, s7, s11

Fewer than you might expect, and more at higher doses. In TRIUMPH-1, 11.3 percent of people on 12 mg stopped because of adverse events, compared with 4.9 percent on placebo.

The TRIUMPH-2 row is odd, with more people quitting on 9 mg than on 12 mg. Single trials bounce around like this, which is a good reason to look at the pattern across trials and not one cell in a table.

TRIUMPH-2 also recorded seven deaths: two on 4 mg, three on 9 mg, one on 12 mg, and one on placebo. The investigators judged none of them related to the study drug.

Sources:s2, s3, s4, s5

Mostly because each report counts people differently. Lilly's May press release put average loss on the 12 mg dose at 28.3 percent after 80 weeks. The NEJM paper's headline figure for the same dose is 25.0 percent, versus 3.9 percent on placebo, and it counts everyone who was randomized, including people who stopped the drug. Counting dropouts tends to pull the average down.

People with type 2 diabetes lose less. Using the same count everyone method, the Lancet paper reports 18.8 percent on 12 mg in TRIUMPH-2, versus 5.1 percent on placebo. Both results are strong human evidence. The lower, count everyone figures sit closer to what happens across a whole group of real people, side effects and dropouts included.

Sources:s1, s2, s3

Probably, because nobody can tell you what is in the vial. Every number in this post comes from Lilly's own drug, made under drug manufacturing rules and started low. In TRIUMPH-2, everyone began at 2 mg a week and stepped up every four weeks, so reaching 12 mg took 16 weeks. In the phase 2 trial, starting at 2 mg instead of 4 mg partly reduced the gut side effects.

Gray market vials skip all of that. No medicine containing retatrutide has been approved by any regulator in the world, Lilly said in August. The FDA's page on unapproved GLP-1 drugs, updated October 1, 2026, says retatrutide cannot be used in compounding under federal law and warns about products falsely labeled for research use that are sold to people. On August 24, the FDA sent warning letters to several peptide sellers, and the one to Royal Peptides lists retatrutide among its unapproved new drugs.

We read through every safety table Lilly has released this year, and none of them describe a vial bought online. If you are weighing it anyway, our guide to reading a COA shows what a real test report looks like, and our post on why a COA matters explains what testing can and cannot catch. The reconstitution calculator helps with the kind of dosing math errors the FDA has linked to hospital visits with compounded semaglutide.

Sources:s4, s6, s8, s9, s10

Lilly plans to submit retatrutide to the FDA in the first quarter of 2027, so approval is unlikely before then. Our earlier post on retatrutide's 2026 data and legal access covers the drug or biologic fight that will decide whether it can ever be compounded.

Here is the side effect picture as of today:

• Gut effects are common, rise with dose, and are the main reason people quit. Strong human evidence.
• Dysesthesia is real, more frequent at higher doses, and mostly mild in Lilly's summary. Strong human evidence that it occurs, limited data on how long it lasts.
• Heart rate and blood pressure shift in some people. Limited human data on how much and for how long.
• Long term heart and kidney safety stays unknown until the outcomes trial reports in 2029.

Our retatrutide entry covers dosing, storage and rarer reported effects. You can compare it with tirzepatide and semaglutide, which have years of approved use behind them. None of this is medical advice, and decisions about these drugs belong with a clinician who knows your history.

Sources:s4, s11

Sources

1. [s1]Jastreboff AM, Kaplan LM, Davies MJ, Wilding JPH, et al.; TRIUMPH-1 Investigators (2026). Retatrutide, a Triple Hormone Receptor Agonist, for Treatment of Obesity (TRIUMPH-1), N Engl J Med, 29 September 2026. rct link
2. [s2]Bellido V, le Roux CW, Ekinci EI, Bjornstad P, Frias JP, et al. (2026). Retatrutide in adults with obesity and type 2 diabetes (TRIUMPH-2): a double-blind, randomised, placebo-controlled, phase 3 trial, Lancet, 29 September 2026. rct link
3. [s3]Eli Lilly and Company (2026). Lilly's triple agonist, retatrutide, delivered powerful weight loss in pivotal Phase 3 obesity trial (TRIUMPH-1 topline, 21 May 2026). manufacturer link
4. [s4]Eli Lilly and Company (2026). Lilly's triple agonist, retatrutide, delivered substantial weight loss and A1C reduction in people with obesity and type 2 diabetes (TRIUMPH-2, 29 September 2026). manufacturer link
5. [s5]Eli Lilly and Company (2026). Lilly's triple agonist, retatrutide, demonstrated significant reductions in A1C and weight in first Phase 3 trial for type 2 diabetes (TRANSCEND-T2D-1, 19 March 2026). manufacturer link
6. [s6]Jastreboff AM, Kaplan LM, Frias JP, et al. (2023). Triple-Hormone-Receptor Agonist Retatrutide for Obesity, a Phase 2 Trial, N Engl J Med 2023. rct link
7. [s7]BioPharma Dive (2026). Lilly, with new data, to seek FDA approval of obesity drug retatrutide (TRIUMPH-2 and TRIUMPH-3 toplines, 23 July 2026). other link
8. [s8]U.S. Food and Drug Administration (2026). FDA's Concerns with Unapproved GLP-1 Drugs Used for Weight Loss (updated 1 October 2026). regulatory link
9. [s9]U.S. Food and Drug Administration (2026). Warning letter: Royal Peptides LLC, MARCS-CMS 734884, 24 August 2026. regulatory link
10. [s10]Eli Lilly and Company (2026). Lilly calls on online platforms, payment companies and regulators to shut down the illegal retatrutide black market (12 August 2026). manufacturer link
11. [s11]Eli Lilly and Company (ClinicalTrials.gov) (2024). The Effect of Retatrutide Once Weekly on Cardiovascular and Kidney Outcomes in Adults Living With Obesity (TRIUMPH-Outcomes), NCT06383390. rct link