The short answer: in Lilly's trials, retatrutide's side effects look like a stronger version of what people get on tirzepatide and semaglutide. Mostly nausea, diarrhea, vomiting and constipation, mostly mild to moderate, mostly while the dose is being raised. Two things stand out that you don't see much on the approved drugs: a skin-sensation side effect called dysesthesia, and more people quitting at the highest dose.
What nobody can tell you yet is whether it's safe over years. The main phase 3 trials ran 80 weeks or less, one subgroup was followed to 104 weeks, and the big heart and kidney outcomes trial doesn't finish until 2029.
Status as of October 30, 2026: Retatrutide is not approved by the FDA or any regulator. Lilly plans to file for U.S. approval in Q1 2027. Every number below comes from Lilly's trials of Lilly's drug. Nothing here is medical advice or a dosing guide.
Want the short version of what's actually approved, legal and tested? Get the free BHN cheat sheet.
Is reta safe?
In the trials, no new type of danger has shown up so far. The TRIUMPH-2 paper in The Lancet called its safety profile "generally consistent with other molecules with GLP-1 receptor agonism" (Lancet, Sept 29, 2026).
That's a narrower statement than "safe." It means:
Short follow-up. The phase 3 trials reported so far ran 40 to 80 weeks. One TRIUMPH-1 subgroup continued to 104 weeks (Lilly, June 6, 2026).
The heart question is still open. TRIUMPH-Outcomes, about 10,000 people, is designed to answer it and isn't expected to finish until February 2029 (ClinicalTrials.gov, NCT06383390).
Trials screen people. Participants were selected, monitored and dose-stepped on a fixed schedule by Lilly's investigators. Real-world use is messier.
It's Lilly's drug. None of this data covers a vial made by someone else. More on that below.
What are the most common reta side effects?
Stomach and gut effects, by a wide margin. Here's how often they showed up at the highest dose tested (12 mg) versus placebo, in each trial Lilly has reported numbers for.
Side effect | TRIUMPH-1 (obesity) | TRIUMPH-2 (obesity + diabetes) | TRIUMPH-4 (obesity + knee arthritis) |
|---|---|---|---|
Nausea | 42.4% vs 14.8% | 28.0% vs 8.0% | 43.2% vs 10.7% |
Diarrhea | 32.0% vs 13.5% | 33.6% vs 13.2% | 33.1% vs 13.4% |
Vomiting | 25.3% vs 4.8% | 15.7% vs 4.2% | 20.9% vs 0.0% |
Constipation | 26.1% vs 10.9% | 16.8% vs 9.4% | 25.0% vs 8.7% |
Dysesthesia (skin sensation) | 12.5% vs 0.9% | 7.3% vs 0.7% | 20.9% vs 0.7% |
Stopped because of side effects | 11.3% vs 4.9% | 7.7% vs 4.9% | 18.2% vs 4.0% |
Lilly describes these events as "generally mild to moderate," with "the majority resolved during treatment." The phase 2 trial found the gut effects were dose-related and "partially mitigated with a lower starting dose," which is why the phase 3 trials started everyone low and stepped up every four weeks (NEJM, 2023).
Why do so many people stop reta in the trials?
Mostly side effects, and it climbs with the dose. In the phase 2 trial, 6% to 16% of people on retatrutide stopped because of side effects, versus none on placebo (NEJM, 2023).
The highest number so far is TRIUMPH-4, at 18.2% on 12 mg. Lilly said those rates were "highly correlated with baseline BMI and included discontinuations for perceived excessive weight loss" (Lilly, Dec 11, 2025). In other words, some people quit because they were losing more than they wanted. Among people who started with a BMI of 35 or more, the 12 mg rate was 12.1%.
The dose pattern isn't perfectly tidy. In TRIUMPH-2, more people stopped on 9 mg (12%) than on 12 mg (8%) (Lancet, 2026).
What is the skin tingling people get on reta?
It's called dysesthesia: an odd, sometimes unpleasant skin sensation, like tingling, burning or sensitivity to touch, with nothing visible on the skin. It's the most distinctive side effect in the retatrutide data.
Phase 2: "cutaneous hyperesthesia and skin sensitivity" in 7% on retatrutide vs 1% on placebo. "None of these events were severe or serious or were associated with overt skin findings" (NEJM, 2023).
Phase 3: from 2.3% to 20.9% across trials and doses, highest in TRIUMPH-4 at 12 mg (table above). Lilly says it was "generally mild and rarely led to treatment discontinuation."
For scale: tirzepatide's label lists dysesthesia at 0.2% to 0.4% vs 0.1% on placebo (Zepbound label, Aug 2026). That's a different drug in different trials, so it's a rough comparison, not a head-to-head.
Nobody has published why it happens.
Does reta raise your heart rate?
Yes, at first, in the trials. The phase 2 paper reported "dose-dependent increases in heart rate" that "peaked at 24 weeks and declined thereafter" (NEJM, 2023). The authors note that glucagon and GLP-1 can both speed up the heart, and called the increases "similar to those reported for GLP-1 receptor agonists."
The same paper reported "cardiac arrhythmia" events, a category that includes things like extra beats and conduction changes, in 3% on placebo and up to 14% in one retatrutide group. They were mild to moderate except one severe case of a prolonged QT interval in someone taking the anti-nausea drug ondansetron.
None of Lilly's phase 3 press releases report heart rate. TRIUMPH-3, in people with existing heart disease, had too few major heart events to answer the question either way: the hazard ratios came with wide uncertainty ranges on both sides of no effect (Lilly, July 23, 2026). That's the job of the 2029 outcomes trial.
Does reta lower blood pressure?
It did in the trials, sometimes too much. In TRIUMPH-2, hypotension (low blood pressure) was reported in 5% on 9 mg and 6% on 12 mg, versus under 1% on placebo (Lancet, 2026).
In the phase 2 trial, blood pressure improved enough that 41% of people in the combined 8 mg group and 30% in the 12 mg group stopped at least one blood pressure medication (NEJM, 2023). That happened under trial supervision. If you're on blood pressure medication, that's the part of this post to bring to your doctor.
Does reta cause hair loss?
There's no published retatrutide number. Hair loss isn't mentioned in any Lilly retatrutide release I read, and it isn't in the phase 2 paper's table of side effects seen in 5% or more of participants (NEJM, 2023). If you've seen a specific hair-loss percentage for retatrutide, I couldn't find it in any trial paper.
What's known about the drug class: tirzepatide's label says hair loss "associated with weight reduction" was reported in 7.1% of women vs 1.3% on placebo, and 0.5% of men vs 0% (Zepbound label). The label ties it to weight loss, and the retatrutide trials produced large weight loss too, so it would be surprising if hair loss never showed up. Until Lilly or a journal reports it, that's reasoning, not data.
Does reta make you tired?
A little more often than placebo, in the one trial that reported it. In phase 2, fatigue was reported in 3% to 12% of the retatrutide groups versus 4% on placebo (NEJM, 2023). Lilly's phase 3 releases don't list fatigue among the most common side effects.
Can reta cause cancer?
Nobody can answer that yet. In the 48-week phase 2 trial, "no cases of ... medullary thyroid cancer, or C-cell hyperplasia were reported" (NEJM, 2023). That's far too short and too small to rule anything out.
For context, approved drugs in this family carry a boxed warning. Tirzepatide's label opens with "WARNING: RISK OF THYROID C-CELL TUMORS," based on findings in rodents (Zepbound label). Whether retatrutide gets the same warning will be the FDA's call after Lilly files.
Can reta make you depressed?
Not in the phase 2 data, though it's small. One person had a depression or suicidal-thoughts event, and that person was on placebo. Nobody in any retatrutide group did (NEJM, 2023). The larger phase 3 papers haven't put mood numbers in their abstracts or press releases.
Reta vs Ozempic side effects: what did the new trial show?
On October 28, Lilly reported the first head-to-head phase 3 trial: retatrutide against Ozempic 2 mg in 1,298 adults with type 2 diabetes, for 80 weeks. The trial was open-label, so everyone knew what they were taking (Lilly via BioSpace, Oct 28, 2026).
Retatrutide 9 mg | Retatrutide 12 mg | Ozempic 2 mg | |
|---|---|---|---|
Diarrhea | 28.3% | 32.7% | 26.4% |
Nausea | 21.2% | 23.8% | 21.3% |
Vomiting | 10.2% | 15.0% | 13.2% |
Constipation | 14.2% | 13.1% | 19.8% |
Dysesthesia | 4.8% | 6.5% | 2.7% |
Stopped because of side effects | 4.8% | 7.9% | 2.7% |
So the gut side effects were roughly in the same range as Ozempic's, with more diarrhea and less constipation. More people stopped on retatrutide. Weight loss was -19.5% and -20.9% on retatrutide vs -9.4% on Ozempic, using Lilly's efficacy estimand (counting people while they stayed on the drug). Full results haven't been published yet.
If you're comparing it with tirzepatide instead, the head-to-head trial, TRIUMPH-5, has no results yet. Its primary completion is estimated for November 2026 (ClinicalTrials.gov, NCT06662383). Why that matters if you're thinking of moving between them: switching from tirzepatide to retatrutide.
Is reta FDA approved?
No. Lilly says it plans to submit a Biologics License Application "in Q1 2027" and is still completing the manufacturing data package the FDA requires (Lilly, Sept 29, 2026). Neither this week's head-to-head release nor Lilly's October 29 earnings report changed that timing (Lilly Q3 2026, SEC filing).
The two legal ways to get it before approval, and who qualifies, are in can you legally get retatrutide.
Does any of this apply to a vial you didn't get from Lilly?
Not directly. Every number above comes from Lilly's own retatrutide, made to its specifications, at known doses, with doctors monitoring. The trials can't tell you what's in a vial from anywhere else, or how much of it.
That's a testing question, not a trial question. What a lab found when it tested thousands of vials is in what's in the vial, and how to read the paperwork that comes with one is in how to read a peptide COA.
The short version
The common side effects are gut effects, mostly mild to moderate, mostly during dose increases.
Dysesthesia, a skin-sensation effect, shows up far more than on approved GLP-1 drugs.
Nearly 1 in 5 people stopped because of side effects at the highest dose in one trial.
Heart rate rose early in phase 2, then came down. The long-term heart answer is due in 2029.
Low blood pressure showed up in about 1 in 20 in one trial. Matters if you're on blood pressure medication.
No published hair-loss number, no long-term cancer data, no long-term anything yet.
Which of these do you want me to dig into when the full papers come out? Reply and tell me. If it's the one you're worried about, that's the one I'll chase first.