The short answer: in the Phase 3 TRIUMPH trials, roughly 2 in 5 participants on the highest dose reported nausea, about 1 in 4 reported vomiting, diarrhoea or constipation, and between 1 in 8 and 1 in 5 reported dysesthesia — an odd, tingling or burning skin sensation that did not appear in the earlier Phase 2 study. Most of these effects were mild to moderate, most clustered around dose escalation, and between 4% and 13% of participants stopped treatment because of them. Retatrutide (LY3437943) is an investigational triple GIP/GLP-1/glucagon receptor agonist from Eli Lilly. It is not approved by any regulator, and no long-term cardiovascular outcome trial has reported. This page sets out what the trials actually measured, at what rate, and what remains genuinely unknown.
What are the most common retatrutide side effects?
Until 2026, the only substantial human safety dataset was a 48-week Phase 2 trial in 338 adults (Jastreboff et al., NEJM, 2023). That has changed. The Phase 3 TRIUMPH programme has now reported from four trials, covering several thousand participants over 68 to 80 weeks. The pattern is consistent across all of them: the side effects are overwhelmingly gastrointestinal, they scale with dose, and they are dominated by nausea.
TRIUMPH-1 is the largest readout — 2,339 adults with obesity or overweight plus at least one weight-related condition, without diabetes, randomised to 4 mg, 9 mg, 12 mg or placebo over 80 weeks. These are the reported rates:
| Adverse event (TRIUMPH-1) | 4 mg | 9 mg | 12 mg | Placebo |
|---|---|---|---|---|
| Nausea | 28.6% | 38.4% | 42.4% | 14.8% |
| Diarrhoea | — | — | 32.0% | — |
| Constipation | — | — | 26.1% | — |
| Vomiting | — | — | 25.3% | — |
| Dysesthesia (altered skin sensation) | — | — | 12.5% | — |
| Discontinued because of an adverse event | 4.1% | 6.9% | 11.3% | 4.9% |
A dash means the figure was not broken out by dose in the public reporting — not that the effect did not occur. The dose gradient in the rows that are broken out is the single most useful thing on this table: nausea rises from 28.6% to 42.4% as the dose triples, and the discontinuation rate nearly triples with it.
TRIUMPH-3 studied a very different population: 1,949 adults with severe obesity (BMI ≥ 35) and established cardiovascular disease — prior heart attack, prior stroke, or symptomatic peripheral arterial disease — with or without type 2 diabetes, randomised to 9 mg, 12 mg or placebo over 80 weeks. It is the trial that matters most for anyone worried about the cardiac signal, and its gastrointestinal rates came out noticeably lower than TRIUMPH-1’s:
| Adverse event (TRIUMPH-3) | 9 mg | 12 mg | Placebo |
|---|---|---|---|
| Diarrhoea | 30.1% | 24.4% | 8.7% |
| Nausea | 21.7% | 22.4% | 5.8% |
| Constipation | 18.0% | 15.7% | 7.1% |
| Decreased appetite | 13.5% | 14.5% | 3.0% |
| Discontinued because of an adverse event | 9.8% | 13.5% | 4.8% |
Three things are worth noticing. First, nausea in TRIUMPH-3 was roughly half the TRIUMPH-1 rate — population and escalation schedule matter, so a single headline number for “the” nausea rate would be misleading. Second, discontinuation was higher in TRIUMPH-3 despite lower gastrointestinal rates, which is a reminder that people stop treatment for reasons a symptom table does not capture. Third, and easily missed: hyperglycaemia was reported less often on retatrutide (3.9% and 3.1%) than on placebo (13.4%), which is what you would expect in a population that includes people with type 2 diabetes.
TRIUMPH-3 also reported cardiometabolic changes in that cardiovascular-disease population: triglycerides down 37.0%, non-HDL cholesterol down 16.5%, systolic blood pressure down 9.3 mmHg, and hsCRP down 51.2%, alongside up to 22.6% weight loss at 80 weeks. Those are risk-factor improvements, not cardiovascular events — the distinction that the next section turns on.
How long do retatrutide side effects last?
The trials point to the same answer the wider incretin literature gives: the gastrointestinal effects are front-loaded around dose escalation and ease with time on a stable dose. In the Phase 2 trial, nausea, vomiting and diarrhoea were described as mostly mild to moderate and clustered during the escalation phase. Reporting from the Phase 3 programme describes the common events as generally mild to moderate, with the majority resolving during treatment rather than persisting to the end of the study.
What the published data do not give you is a clean per-symptom duration curve — no trial has reported “median time to resolution of nausea” for retatrutide. Anyone quoting a specific number of days or weeks for how long side effects last is extrapolating, not citing. The honest version is: most fade, some do not, and the trials have not measured it precisely enough to say more.
Dysesthesia: the side effect that only appeared in Phase 3
Dysesthesia — a distorted sense of touch, usually described as tingling, prickling or burning skin that is out of proportion to what is touching it — is the one genuinely new finding of the Phase 3 programme. It was not reported in the Phase 2 trial at all.
| Dysesthesia rate | 9 mg | 12 mg | Placebo |
|---|---|---|---|
| TRIUMPH-4 (obesity + knee osteoarthritis, 68 weeks) | 8.8% | 20.9% | 0.7% |
| TRIUMPH-1 (obesity, 80 weeks) | — | 12.5% | — |
The dose relationship is stark: at 12 mg in TRIUMPH-4, dysesthesia was reported roughly thirty times more often than on placebo, and more than twice as often as at 9 mg. It has been linked in commentary to retatrutide’s glucagon-receptor activity — the component that distinguishes it from GLP-1-only and GLP-1/GIP compounds. That link is a hypothesis, not an established mechanism. Reported cases were characterised as generally mild to moderate, but a symptom that emerges only in longer, larger trials is exactly the kind of finding that argues for caution about what else longer follow-up might surface.
What does retatrutide do to heart rate?
The Phase 2 trial recorded a dose-dependent rise in heart rate of up to about 6.7 beats per minute, which peaked around week 24 and then declined. Reviewers also noted reports of mild-to-moderate cardiac arrhythmia during Phase 2 development, and called explicitly for dedicated long-term cardiovascular outcome trials before the compound’s cardiovascular safety could be considered settled (Ray, 2023; Doggrell, 2023).
That call has been partly answered and mostly not. TRIUMPH-3 did enrol 1,949 people with established cardiovascular disease and ran for 80 weeks without a safety stop, which is meaningful reassurance about short-to-medium-term tolerability in exactly the population that would be most vulnerable. But its primary endpoint was percent change in body weight, not cardiovascular events. It reported improvements in risk markers — blood pressure, triglycerides, hsCRP — which are not the same thing as fewer heart attacks and strokes.
A trial designed to answer “does this reduce or increase cardiovascular events over years?” has not reported. Until one does, the accurate status of retatrutide’s cardiovascular profile is encouraging on markers, unresolved on outcomes.
How many people stop because of side effects?
This is the most practical single measure of tolerability, because it captures the effects people actually found intolerable rather than the ones they merely reported. Across the two trials with published figures, discontinuation attributed to adverse events ran at 4.1% to 11.3% in TRIUMPH-1 and 9.8% to 13.5% in TRIUMPH-3, against 4.8–4.9% on placebo. In both trials, the highest dose roughly doubled to tripled the placebo rate.
Read the other way, that means the large majority of participants — around 87% to 96% depending on dose and trial — stayed on treatment. Both halves of that sentence are true and neither should be quoted without the other.
Who is at higher risk?
Because retatrutide is unapproved, no regulator has published a contraindication list. What the literature supports:
- People with a history of arrhythmia. The dose-dependent heart-rate increase and the Phase 2 arrhythmia signal are the clearest reason for caution. TRIUMPH-3 ran 80 weeks in a cardiovascular-disease population without a safety stop, but no trial has yet used cardiovascular events as its primary endpoint.
- Anyone at the higher doses. Every category — nausea, dysesthesia, discontinuation — rises with dose. The 12 mg arm is not a scaled-up version of the 4 mg experience; it is a materially different tolerability profile.
- Anyone escalating quickly. The trials used slow, structured escalation. The gastrointestinal burden concentrates in exactly that window, and it is the one variable the trials deliberately controlled.
- Class-based cautions not yet characterised for retatrutide. Approved GLP-1-based drugs carry warnings around pancreatitis, gallbladder events and, in rodents, thyroid C-cell tumours. Whether these apply to retatrutide specifically is unknown. Absence of data is not evidence of absence.
What we still do not know
- Long-term safety beyond about two years. The longest TRIUMPH readouts run to 80 weeks with an extension. Nothing published tells you what five years looks like.
- Whether the heart-rate and arrhythmia signal translates into hard cardiovascular events. No dedicated cardiovascular outcome trial has reported.
- What causes dysesthesia, whether it fully resolves, and whether it persists after stopping. It is a Phase 3 finding with no mechanistic study behind it yet.
- Muscle mass, bone and joint effects, and weight regain after discontinuation. These are raised as concerns for incretin-based weight-loss compounds generally and have not been quantified for retatrutide specifically.
Why trial safety data do not transfer to a research vial
Every number on this page came from a controlled trial using pharmaceutical-grade material of verified identity, purity and concentration, administered on a fixed escalation schedule with medical monitoring. A vial of “research-grade” retatrutide from a peptide vendor shares none of those conditions. Its actual content is unverified unless a third-party certificate of analysis says otherwise, and even then the trial safety profile describes the drug, not the vial.
Anecdotal reports from online research-peptide communities are not a safety dataset for the same reason: unverified product, unverified dose, no controls, and no systematic adverse-event capture. They should not be weighed against trial data.
Dosage and handling reference
For documenting quantities, reconstitution volumes and concentrations in research records, see our retatrutide dosage calculator, the retatrutide 12 mg vial protocol, and the general peptide reconstitution guide. These are laboratory documentation tools only — not human-use instructions, dosing recommendations, or a suggestion to self-administer. Retatrutide is an investigational compound; any decision involving it belongs with a qualified healthcare professional.
Frequently Asked Questions
What is the most common side effect of retatrutide?
Nausea. In the Phase 3 TRIUMPH-1 trial it was reported by 28.6% of participants at 4 mg, 38.4% at 9 mg and 42.4% at 12 mg, against 14.8% on placebo. Diarrhoea, constipation and vomiting follow, each in the 24–32% range at the highest dose. All were described as mostly mild to moderate.
Does retatrutide have long-term side effects?
Nobody knows yet. The longest Phase 3 readouts run to 80 weeks plus an extension, and no trial with cardiovascular events as its primary endpoint has reported. Effects beyond roughly two years — including on muscle mass, bone, and weight regain after stopping — have not been measured for retatrutide.
Does retatrutide cause heart palpitations or affect the heart?
Phase 2 trials recorded a dose-dependent heart-rate increase of up to about 6.7 beats per minute, peaking near week 24 then declining, alongside reports of mild-to-moderate cardiac arrhythmia. Phase 3 TRIUMPH-3 then ran 80 weeks in 1,949 people with established cardiovascular disease and reported improvements in blood pressure, triglycerides and hsCRP. That is reassuring on risk markers, but no trial has yet had cardiovascular events as its primary endpoint, so the long-term question is unresolved.
What is dysesthesia and how common is it with retatrutide?
Dysesthesia is an altered, often tingling or burning skin sensation. It appeared only in Phase 3: 20.9% at 12 mg and 8.8% at 9 mg in TRIUMPH-4 versus 0.7% on placebo, and 12.5% at 12 mg in TRIUMPH-1. It was absent from the Phase 2 report. Cases were characterised as generally mild to moderate.
How long do retatrutide side effects last?
Gastrointestinal effects concentrate around dose escalation and ease on a stable dose; the Phase 3 reporting describes most common events as resolving during treatment. No trial has published a median time to resolution, so any specific “it lasts X weeks” figure is an extrapolation rather than a measured result.
How many people stop retatrutide because of side effects?
In TRIUMPH-1, 4.1%, 6.9% and 11.3% at 4, 9 and 12 mg respectively, against 4.9% on placebo. In TRIUMPH-3, 9.8% at 9 mg and 13.5% at 12 mg against 4.8% on placebo. The rate rises with dose in both trials.
Is retatrutide safe?
There is not enough evidence to call it safe. Phase 3 data show a tolerability profile that is manageable for most participants at lower doses and demanding at 12 mg, plus an unexplained dysesthesia signal and an unresolved cardiovascular question. It is not approved by any regulator. “Short-term profile characterised, long-term safety unknown” is the accurate description.
This page is a research-use-only reference and is not medical advice. It summarises published literature, trial registry records and company trial reports. Consult a qualified healthcare professional for any medical decision.
References
- Jastreboff AM, Kaplan LM, Frías JP, et al. Triple–Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial. N Engl J Med. 2023;389(6):514–526. doi:10.1056/NEJMoa2301972 (PMID 37366315; NCT04881760).
- Eli Lilly and Company. Lilly’s triple agonist, retatrutide, delivered powerful weight loss in pivotal Phase 3 obesity trial (TRIUMPH-1). Press release, 21 May 2026. investor.lilly.com.
- Eli Lilly and Company. Lilly’s triple agonist, retatrutide, successful in two additional Phase 3 obesity trials (TRIUMPH-2 and TRIUMPH-3). Press release, July 2026. investor.lilly.com.
- Eli Lilly and Company. Lilly’s triple agonist, retatrutide, delivered weight loss along with substantial relief from osteoarthritis pain in first successful Phase 3 trial (TRIUMPH-4). Press release. investor.lilly.com.
- BioSpace. Lilly’s retatrutide scores triple trial triumph — but a new safety signal emerges (dysesthesia rates, TRIUMPH-4). 2026. biospace.com.
- ClinicalTrials.gov. TRIUMPH-1: A Study of Retatrutide (LY3437943) in Participants With Obesity or Overweight. NCT05929066. clinicaltrials.gov.
- ClinicalTrials.gov. TRIUMPH-3: Retatrutide in Participants With Severe Obesity and Established Cardiovascular Disease (primary endpoint: percent change in body weight). NCT05882045. clinicaltrials.gov.
- Retatrutide for the treatment of obesity, obstructive sleep apnea and knee osteoarthritis: rationale and design of the TRIUMPH registrational clinical trials. 2025. PMC12673447 (PMID 41090431).
- Drug Discovery World. Phase III trials confirm benefits of Lilly’s triple agonist retatrutide (TRIUMPH-2 and TRIUMPH-3 populations and results). July 2026. ddw-online.com.
- Wu X, Yang Y, Cui X, et al. Efficacy and safety of incretin-based therapies in type 2 diabetes: a network meta-analysis. Front Pharmacol. 2026;17:1846714. doi:10.3389/fphar.2026.1846714 (PMID 42394981).
- Takrori E, Peshin S, Singal S. Gastrointestinal Adverse Effects of Anti-Obesity Medications in Non-Diabetic Adults: A Systematic Review. Medicina (Kaunas). 2025;61(11):1987. doi:10.3390/medicina61111987 (PMID 41303824).
- Abulehia A, et al. Comparative Efficacy and Safety of Glucagon Receptor Agonists: A Network Meta-Analysis. Endocrinol Diabetes Metab. 2026;9(2):e70187. doi:10.1002/edm2.70187 (PMID 41787737).
- Ullah MI, Tamanna S. Obesity: Clinical Impact, Pathophysiology, Complications, and Modern Innovations in Therapeutic Strategies. Medicines (Basel). 2025;12(3):19. doi:10.3390/medicines12030019 (PMID 40843857).
