Retatrutide side effects: what the trials actually found

Last reviewed 7 September 2026 by retainfo editorial team. How we review content.

Everything on this page comes from trials of the real drug, given in monitored settings with scheduled dose steps. The most common effects were stomach-related and mild to moderate. A few were new or unexpected. Products sold online carry all of these risks plus others that the trials could not measure, because the trials did not use them.

The headline numbers

Nausea affected 28.6% to 42.4% of people on retatrutide in TRIUMPH-1, depending on dose, versus 14.8% on placebo.^[1]
Dysesthesia (tingling, burning or heightened skin sensation) is new to this drug: up to 12.5% in TRIUMPH-1 and 20.9% at the top dose in TRIUMPH-4.^[2]
Heart rate rose in a dose-dependent way in Phase 2, peaking around week 24 and then declining. Arrhythmias were reported in 2% to 11% of retatrutide participants versus 2% on placebo.^[3]
Stopping because of side effects: 4.1%, 6.9% and 11.3% at 4, 9 and 12 mg versus 4.9% on placebo in TRIUMPH-1.
Rare but serious in trials: one case of acute pancreatitis in Phase 2; transient liver-enzyme rises in about 1%.
Not from trials: a cluster of acute liver injuries in Australia in people using products labelled retatrutide, suspected to involve a contaminant.^[4]

Side effects by dose

TRIUMPH-1: the most common side effects at 80 weeks, by trial arm. Figures are from Lilly's 21 May 2026 topline announcement; the peer-reviewed paper may differ slightly.
Event	4 mg	9 mg	12 mg	Placebo
Nausea	28.6%	38.4%	42.4%	14.8%
Diarrhoea	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 (altered skin sensation)	5.1%	12.3%	12.5%	0.9%
Upper respiratory tract infection	14.2%	12.2%	13.1%	11.6%
Urinary tract infection	7.5%	8.8%	8.4%	5.3%
Stopped treatment because of side effectsAny adverse event leading to discontinuation	4.1%	6.9%	11.3%	4.9%

Two things stand out. First, every gastrointestinal effect rises with dose. Second, placebo rates are not zero: a sixth of people on dummy injections reported nausea, which is a reminder that expectation, diet changes and the injection itself all contribute.

In detail

1.Nausea, vomiting and gut effects of retatrutide
How often retatrutide caused nausea, vomiting, diarrhoea, constipation and reflux in trials, when it was worst, and when gut symptoms become dangerous.
2.Retatrutide, heart rate and heart rhythm
Retatrutide raised resting heart rate in a dose-dependent way in Phase 2 and arrhythmias were more common. What is known, who should be cautious, and why.
3.Dysesthesia: retatrutide's new skin-sensation effect
Tingling, burning and oversensitive skin affected up to one in five people on retatrutide in Phase 3 trials. What it is, how common, and what is unknown.
4.Pancreatitis and gallbladder risk with retatrutide
Pancreatitis and gallstones are known risks of the GLP-1 class. What retatrutide's trials found, what the SmPC warnings say, and the symptoms to act on.
5.Retatrutide and the liver and kidneys
In trials retatrutide cut liver fat and rarely disturbed liver tests; the 2026 liver-injury cluster involved unlicensed products. Kidney risk: dehydration.
6.How many people stopped retatrutide in the trials?
Stopping because of side effects ran from 2% to 18% across retatrutide's Phase 3 trials versus 0% to 5% on placebo. What the figures do and do not mean.
7.Retatrutide: what is still unknown
An honest list of the gaps in retatrutide's safety evidence: long-term heart outcomes, pregnancy, older people, kidney and liver disease, regain, and more.
8.When to seek help after using retatrutide
The red-flag list: which symptoms after an injected product need 999, NHS 111 today, a GP this week or a Yellow Card report, and what to tell them.

When to seek urgent help

Get medical help the same day if you have any of these after using any injected product:

Severe or persistent tummy pain, especially spreading to the back, with or without vomiting
Yellow eyes or skin, dark urine, pale stools, itching, or unusual bruising
Vomiting that stops you keeping fluids down for more than a day, or signs of dehydration
A racing, pounding or irregular heartbeat, fainting, or chest pain
Redness, swelling, heat or pus at an injection site, or a fever after injecting

Call 999 or go to A&E if someone is struggling to breathe, has chest pain, is confused or drowsy, has a fit, or collapses.
Call NHS 111 (or use 111 online) if you are unwell and unsure what to do. Say exactly what you used, when, and how much.
See your GP for anything that is persistent or worrying but not an emergency. Tell them honestly what you have taken. See how to talk to your GP.
Report it. Any suspected reaction to any product, licensed or not, can be reported to the MHRA Yellow Card scheme. See how to report a reaction.

References

Lilly's triple agonist, retatrutide, delivered powerful weight loss in pivotal Phase 3 obesity trial (TRIUMPH-1 topline results). Eli Lilly and Company, press release, 21 May 2026. Primary source
Lilly's triple agonist, retatrutide, delivered weight loss of up to an average of 71.2 lbs along with substantial relief from osteoarthritis pain in first successful Phase 3 trial (TRIUMPH-4 topline results). Eli Lilly and Company, press release, 11 December 2025. Primary source
Jastreboff AM, Kaplan LM, Frías JP, et al. Triple–Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial. New England Journal of Medicine 2023;389:514-526, 26 June 2023. Primary source
Toxicity linked to unapproved peptide product labelled Retatrutide (health alert). Victorian Department of Health (health.vic.gov.au), 19 June 2026. Primary source

Side effects by dose

In detail

References

Related pages