Retatrutide Diarrhea: How Common It Is and How to Manage It

Yes, retatrutide diarrhea is common. In TRIUMPH-1, Lilly's 80-week Phase 3 obesity trial, about 1 in 3 people on 9 mg (34.1%) had diarrhea at some point, against 13.5% on placebo. It bunches up around dose increases and mostly stops rising past 9 mg. In the first Phase 3 head-to-head trial it was a little more common than on Ozempic 2 mg: 32.7% on retatrutide 12 mg against 26.4%.

Diarrhea gets less attention than nausea, yet in TRIUMPH-2, TRIUMPH-3 and TRANSCEND-T2D-2 it was reported more often than nausea at every dose tested. The numbers below come from Lilly's trial releases as of October 2026. TRIUMPH-2's paper in The Lancet gives the same rates, rounded.

How common is retatrutide diarrhea in the trials?

Lilly has released diarrhea rates for all 6 Phase 3 trials that have reported, which randomized 7,720 people between them (adding up Lilly's figures for each trial). Everyone on retatrutide started at 2 mg and stepped up every 4 weeks to their assigned dose, so the doses line up across trials even though the people in them differ.

Placebo matters more here than for most side effects, because diarrhea is common anyway: 13.5% of people on placebo in TRIUMPH-1 had it too. Subtract each trial's placebo rate and 9 mg lands in a tight band, 20.3 to 21.8 extra people in every 100, whoever was in the trial. That's our arithmetic from Lilly's figures, and it's the fairer way to size the risk.

Is retatrutide diarrhea worse than Ozempic?

Slightly, in the only trial that has tested it. TRANSCEND-T2D-2 randomized 1,298 adults with type 2 diabetes to retatrutide 9 mg, retatrutide 12 mg or Ozempic 2 mg for 80 weeks, and Lilly released the topline on October 28, 2026. Diarrhea hit 28.3% on 9 mg and 32.7% on 12 mg, against 26.4% on Ozempic. That's about 2 extra people in every 100 at 9 mg and about 6 at 12 mg.

The rest of the gut picture was mixed. Nausea was level at 9 mg (21.2% against 21.3%), and constipation was less common on retatrutide, 14.2% and 13.1%, than on Ozempic at 19.8%. Weight loss was more than double: 20.9% on 12 mg against 9.4% on Ozempic (efficacy estimand, as if everyone stayed on treatment).

The trial was open-label, so everyone knew which drug they were on, and there's no journal paper yet, only Lilly's release.

Other comparisons are indirect. The FDA label for tirzepatide (Zepbound) lists diarrhea in 19%, 21% and 23% of people on 5, 10 and 15 mg, against 8% on placebo, and the Wegovy 2.4 mg label lists 30% against 16%. Those come from different trials and people, but retatrutide sits at or somewhat above that range.

Diarrhea mostly levels off after 9 mg, and a slow start helps less

Nausea climbs with every dose. In TRIUMPH-1 it went from 28.6% on 4 mg to 38.4% on 9 mg to 42.4% on 12 mg, which our retatrutide nausea page covers in full. Diarrhea mostly didn't follow that pattern. It rose from 4 mg to 9 mg, then stopped: in 4 of the 6 trials in the table, the 12 mg group had less diarrhea than the 9 mg group, and in TRIUMPH-2 the two were within 0.1 points. TRANSCEND-T2D-2 is the exception, with 12 mg 4.4 points above 9 mg.

The gaps between 9 mg and 12 mg are small and Lilly didn't report a statistical test for them, so read this as a pattern rather than proof.

The Phase 2 trials show the same flattening: in both, the 12 mg group had less diarrhea than either 8 mg group. Both also tested a 2 mg first dose against 4 mg. For nausea it made a big difference: in the obesity trial (NEJM), registry results show nausea at an 8 mg target falling from 60.0% to 17.1% with the lower start. Diarrhea didn't move.

In the diabetes trial, published in The Lancet, the lower start did come with less diarrhea. But the 4 mg and 8 mg groups there had only 23 to 26 people each, so a difference of 4 people changes the percentage by more than 15 points.

Our read: in 5 of the 6 Phase 3 trials, the diarrhea risk had mostly arrived by 9 mg, and a gentler ramp is a weaker lever for diarrhea than it is for nausea.

Why does reta cause diarrhea?

Nobody has published the mechanism for retatrutide specifically. It activates three hormone receptors: GLP-1, GIP and glucagon. Drugs that share the GLP-1 signal cause diarrhea as well (see the Ozempic, tirzepatide and Wegovy figures above), so part of it is class behavior. The trials don't separate how much each of the three signals adds.

The labels for tirzepatide and semaglutide say both drugs slow how fast the stomach empties. That's an easy fit for nausea and feeling full, and a harder one for loose stools. We couldn't find a primary source that explains the diarrhea, so we won't pretend it's settled.

Forum posts are user reports, not a study. A JAMA Network Open analysis of 148,640 retatrutide posts and comments on Reddit (October 2026) found the effects users mentioned most were increased appetite (17.1%), fatigue (15.2%), more energy, nausea, food cravings and insomnia. Diarrhea isn't on that list. Those posters were using unverified products, so it can't tell you how often the real drug causes it. We go through more of what people post on our retatrutide Reddit page.

Does reta make you poop more?

It depends which way your gut goes. In TRIUMPH-1, constipation affected 23.8% to 26.1% of people on retatrutide, against 10.9% on placebo, close to the diarrhea rates. Some people get one, some the other, and the trial summaries don't say how many had both. In TRANSCEND-T2D-2, constipation was less common on retatrutide (13.1% to 14.2%) than on Ozempic 2 mg (19.8%), while diarrhea was more common.

No trial counted bowel movements per week, so there's no average change to report.

When does it start and how long does it last?

Lilly reported that stomach side effects in TRANSCEND-T2D-1 came mainly during dose escalation. Lilly's patient site for tirzepatide says the same about that drug: most of the nausea, diarrhea and vomiting in its studies came with dose increases and generally eased over time.

So the weeks after each step are when to expect it. For someone aiming at 12 mg in the Phase 3 design, that's four increases, one every 4 weeks, with the last at the start of week 17. None of the retatrutide results we reviewed give a day count for a typical bout, or report diarrhea severity on its own.

For comparison, the NHS says diarrhea from a stomach bug usually stops within 5 to 7 days. Diarrhea that keeps going on a steady dose, weeks after your last increase, doesn't fit the trial pattern and deserves a call to your doctor. Timelines for other side effects are on our page about how long retatrutide side effects last.

Can stopping retatrutide cause diarrhea?

No published trial reports diarrhea caused by stopping retatrutide. What is known is how slowly it leaves: its half-life is about 6 days, per the Phase 1b trial in The Lancet. Our retatrutide half-life page works out how long it takes to wear off after the last shot.

Our read: because roughly half the drug is still around after 6 days, diarrhea in the first couple of weeks after a last shot can still be the drug at work, while diarrhea that starts much later more likely has another cause. Either way, the NHS threshold applies: more than 7 days of diarrhea is worth getting checked.

How to stop diarrhea on retatrutide

Replace what you lose

Lilly's diarrhea tips for tirzepatide start with fluids: at least 8 to 10 glasses of clear fluids a day, with water preferred. The NHS adds two useful details. Skip fruit juice and fizzy drinks, which can make diarrhea worse. And if you're losing a lot of fluid, a pharmacist can recommend an oral rehydration solution, a powder you mix with water that puts back the salts and sugar plain water doesn't.

Keep an eye on your pee. Dark yellow, strong-smelling pee or peeing less often than usual are signs of dehydration, per the NHS.

Change what you eat

Lilly suggests limiting dairy and eating bland food like chicken broth, rice or carrots. The NHS says it may help to avoid fatty or spicy food.

Ask about a short-term medicine

The NHS says a pharmacist may suggest a medicine such as loperamide to stop diarrhea for a few hours. No retatrutide trial has tested it alongside the drug, so ask a pharmacist or doctor first and tell them you're on retatrutide. Don't use it to hide diarrhea that comes with the warning signs in the next section.

Look at the dose, with your doctor

The semaglutide label tells prescribers to consider delaying the next increase by 4 weeks when a dose isn't tolerated, and the tirzepatide label suggests a lower maintenance dose in the same situation. Our dosing schedule page covers when holding a step makes sense. For retatrutide, the first table adds a twist: in 5 of the 6 trials, 12 mg brought no more diarrhea than 9 mg, give or take 0.1 points, so stepping back from 12 mg may help nausea more than it helps diarrhea.

Dose accuracy counts too. Lilly says products sold as retatrutide outside its trials may hold too much or too little active ingredient, or the wrong one. If you're weighing retatrutide for sale, a batch-specific third-party lab report is how you'd check that the milligrams on the label are the milligrams in the vial.

Protect your contraception

The NHS says that if you have diarrhea for more than 24 hours on the combined pill, keep taking it but use condoms or another method until 7 days after the diarrhea stops. The tirzepatide label separately advises a backup or non-oral method for 4 weeks after starting and after each dose increase. Lilly completed a study of retatrutide with an oral contraceptive in postmenopausal women (NCT06039826) in July 2024, but as of October 2026 no results have been posted, so nobody outside Lilly knows how retatrutide affects the pill. More on birth control and pregnancy planning is on our retatrutide and pregnancy page.

When diarrhea needs a doctor

Fluid loss is what turns diarrhea serious. The UK medicines regulator, the MHRA, says diarrhea, nausea and vomiting from GLP-1 medicines sometimes lead to severe dehydration that needs hospital care. The tirzepatide label says most reported cases of acute kidney injury happened in people dehydrated by these same side effects, and in the Phase 2 retatrutide obesity trial, 1 of 35 people in one 8 mg group had a serious acute kidney injury.

Two other conditions can hide behind a bad stomach week. The MHRA describes pancreatitis as severe stomach pain that spreads to the back and won't go away. And a June 2026 health alert from Victoria, Australia, reported 6 cases of acute liver injury in people using unapproved products labeled Retatrutide, with belly pain, yellow skin, dark urine and unusual bruising.

For the wider safety record, see is retatrutide safe.