Diarrhea on GLP-1 medications

Where diarrhea appears in the labels

In the Mounjaro® prescribing information, diarrhea is the second row of Table 1 in section 6.1, right after nausea, and the Medication Guide lists it among the most common side effects ^[1][2]. Diarrhea also appears in the section 6.1 tables and Medication Guides of Zepbound®, which contains the same medicine, tirzepatide, and of the semaglutide labels for Ozempic® and Wegovy® ^[3][4][5][6].

Each label also warns, in section 6.1, that rates from different trials cannot be directly compared and may not reflect everyday practice. Treat the numbers below as descriptions of trial groups.

Trial rates against placebo

Each row is the US label’s main placebo-controlled pool. Ranges cover the dose groups tested. The Zepbound® count also includes reports of frequent bowel movements.

A closer look at the Mounjaro label

The Mounjaro® pool covered 718 adults on the medicine for an average of 36.6 weeks. Counting every stomach and bowel reaction together, 37.1% to 43.6% of people on Mounjaro® reported one, against 20.4% on placebo, and 3.0% to 6.6% stopped treatment because of them, against 0.4% on placebo ^[1].

The label adds that the majority of nausea, vomiting and diarrhea reports occurred during dose escalation and decreased over time. The Zepbound® label says the same about its weight-reduction trials. In the Mounjaro® trial of children aged 10 and older with type 2 diabetes, reactions were described as consistent with adults, except for more vomiting, abdominal pain and low blood sugar ^[1][3].

What the European and Canadian product information adds

The European Medicines Agency lists diarrhea as “very common”, meaning at least 1 person in 10, for Mounjaro®, Wegovy® and Ozempic® ^[7][8][9]. There is no European Zepbound® document because tirzepatide is sold in the EU only as Mounjaro®, for diabetes and for weight management; the Canadian Zepbound® monograph gives its own figures.

How long it lasts and what a trial analysis found

The US labels give no duration. The European Wegovy® information does: across its trials the median bout of diarrhea lasted 3 days, compared with 8 days for nausea and 47 days for constipation ^[8].

A pooled analysis of the STEP 1 to 3 trials of semaglutide 2.4 mg reported diarrhea in 29.7% versus 15.9% on placebo. Stomach and bowel events on semaglutide were mostly mild to moderate and transient, and weight loss was much the same in people with and without them, so diarrhea is not a sign that the medicine is working better. The analysis was manufacturer-funded and covers semaglutide only ^[11].

Why the labels connect diarrhea to the kidneys

All four labels include a section 5 warning on acute kidney injury due to volume depletion. They report that most cases happened in people whose stomach reactions, such as nausea, vomiting or diarrhea, led to dehydration, and that kidney function should be monitored in people reporting such reactions, especially when treatment starts and when the dose goes up. In the Zepbound® trials, acute kidney injury was reported in 0.5% of people on the medicine and 0.2% on placebo ^[1][3][5][6].

The Zepbound® label also notes that low blood pressure occurred in 1.6% of people on the medicine versus 0.1% on placebo, and that it occurred in association with stomach reactions and dehydration as well ^[3].

The NIH lists the signs of dehydration to look for: extreme thirst or a dry mouth, passing less urine or dark urine, feeling tired, dizzy or lightheaded, sunken eyes or cheeks, and skin that is slow to flatten after a gentle pinch ^[12].

Why it may happen

What the labels support: the labels report how often diarrhea happened and that it clustered around dose increases, but they do not explain the cause. They describe a delay in stomach emptying, which for tirzepatide is largest after the first dose and shrinks over time ^[1][3].

What is proposed but not measured: it is plausible that the medicines’ effects on the digestive tract change bowel habits in both directions, which would fit diarrhea and constipation both appearing in the same tables. No label states a mechanism for diarrhea.

General medical background: the NIH names medicine side effects as one of the most common causes of sudden diarrhea, alongside viral stomach infections and food poisoning ^[12].

What else could it be

Not every bout of diarrhea is the medicine. The NIH lists viral gastroenteritis, food poisoning and travellers’ diarrhea; other medicines, such as antibiotics and antacids containing magnesium; and sugar alcohols such as sorbitol and xylitol, found in some sugar-free products and liquid medicines ^[12].

Longer-lasting diarrhea can come from food intolerances, such as lactose or fructose intolerance, from food allergies and from digestive conditions ^[12]. If diarrhea started with a new food, a trip or a new medicine, or if others who ate the same meal are ill, write that down too.

What may help

The Medication Guides give one practical tip: drinking fluids helps reduce the chance of dehydration ^[2][4]. General NIH guidance and the manufacturer’s patient page say more; neither replaces your prescriber’s advice.

• Replace fluids and electrolytes with water, broths or oral rehydration solutions. If you have diabetes or kidney disease, the NIH asks you to talk with a doctor before using rehydration solutions, as a specific type or amount may suit you better ^[13].
• Avoid drinks with caffeine, foods and drinks high in simple sugars or sugar alcohols, high-fat food such as fried food, and milk products containing lactose while diarrhea lasts ^[14].
• When your appetite comes back, the NIH says most people can return to their normal diet; it does not advise fasting or a strict diet ^[14].
• Eli Lilly’s Zepbound® page suggests drinking plenty of clear fluids, limiting dairy and trying bland foods such as broth, rice or carrots ^[15].
• The NIH names loperamide and bismuth subsalicylate as over-the-counter options for short-term diarrhea, and says to see a doctor if it worsens or lasts more than 2 days while taking them. Ask your pharmacist whether either suits you alongside your other medicines ^[13].

What to write down

For diarrhea, counts help most: how many loose stools a day, the date and time it started, how many days after your injection that was, and how many days it lasted. Note what and when you ate beforehand, how much you drank, and whether nausea, vomiting or stomach pain came with it. Mark the weeks right after a dose increase, the period the label points to.

The NIH also suggests a food and drink journal for diarrhea that keeps coming back, noting which items seem to make it worse ^[14]. Together those notes let your clinician see whether diarrhea is tied to a point in your dosing week, is fading as the label describes, or is lasting long enough to matter for fluids.

Mounjaro® and Zepbound® are registered trademarks of Eli Lilly and Company. Ozempic® and Wegovy® are registered trademarks of Novo Nordisk A/S. PepLabo is not affiliated with or endorsed by Eli Lilly and Company or Novo Nordisk.

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