GLP-1 Diarrhea: Why It Happens, How Long It Lasts and What Helps

Diarrhea on Mounjaro, Zepbound, Ozempic or Wegovy is common, usually mild and usually short. In the trials, each bout lasted a median of three days, and most happened while the dose was going up. What helps: smaller, lower-fat meals, fewer of the foods that loosen stools (sugar alcohols, caffeine, alcohol, fried or spicy food), psyllium rather than bran if you take a fiber supplement, and plenty of fluids. Diarrhea that lasts more than a week, has blood in it or leaves you dehydrated needs a doctor, not a diet change.

How common it is

In the STEP trials of semaglutide 2.4 mg (Wegovy), 29.7% of people had diarrhea at some point, against 15.9% on placebo. On tirzepatide (Zepbound) it was 19 to 23% depending on the dose, against 8% on placebo, and the label notes that most episodes happened during dose escalation and decreased over time. Mounjaro is the same medicine as Zepbound, and Ozempic the same medicine as Wegovy, at lower doses.

Almost all of it was mild to moderate. Across the STEP trials, 99.5% of stomach side effects were non-serious.

Why GLP-1s cause diarrhea

The honest answer is that nobody knows exactly. The authors of the STEP analysis write that changes in how fast food moves through the lower gut may cause both constipation and diarrhea, "although evidence is lacking". A 2023 expert consensus notes that the long-acting, once-weekly medicines are linked with less nausea but more diarrhea than older daily ones, possibly because they act on the gut for longer.

What you eat matters too. The 2025 joint advisory from four US obesity and nutrition societies says diarrhea improves when you avoid large or high-fat meals. And eating on a GLP-1 often brings new foods into your day, such as protein bars, sugar-free snacks and milk-based shakes, some of which are known to loosen stools.

It isn't a sign the medicine is working. In the STEP trials, people without stomach side effects lost nearly as much weight as people with them, and less than one percentage point of the weight loss was explained by side effects.

How long it lasts

• Each bout: a median of three days in the semaglutide trials, about the same as on placebo.
• When it starts: usually in the first four weeks, after which new cases drop off noticeably.
• Over the months: in the STEP trials, diarrhea was most common around week 20, just after people reached the top dose, and became less common after that. A dose increase can bring a short flare.

The NHS says diarrhea usually stops within five to seven days. If yours lasts longer than seven days, get advice (see below).

What to eat

You don't have to live on toast. The NIH says that with short-term diarrhea you can eat your normal diet, minus the foods that make it worse. On a GLP-1 that means:

• Small, lower-fat meals. Grill, bake or poach rather than fry, and stop at the first sign of fullness.
• Gentle protein at every meal. Eggs, chicken, turkey, fish, tofu and lean mince. Don't let protein slide just because your stomach is unsettled; see protein targets on a GLP-1.
• Easy carbohydrates. Rice, potatoes, oats, bananas and toast are simple to eat when your appetite is low.
• Psyllium, not bran. Psyllium, a gel-forming fiber, holds on to water in the bowel and helps normalize stool form in both constipation and diarrhea, while coarse wheat bran tends to speed things up. Add fiber gradually and drink more as you do; see fiber on a GLP-1.
• A shake if food won't go down. Choose one without sugar alcohols, and try a whey isolate or lactose-free milk if dairy seems to set you off. More in protein shakes on a GLP-1.

Lower-fat, high-protein meals from the site:

• Teriyaki Chicken Bowl: 40 g protein, 11 g fat
• Chicken and White Bean Soup: 30 g protein, 5 g fat
• Egg White Veggie Frittata: 24 g protein, 11 g fat

Foods and drinks that make it worse

The NIH advises avoiding these while you have diarrhea, and the NHS adds fatty and spicy food:

• Fried and fatty food, such as fried food, pizza and fast food. On a GLP-1, large or rich meals are the most common trigger.
• Caffeine, in coffee, tea and some soft drinks; see coffee on a GLP-1.
• Alcohol; see alcohol on a GLP-1.
• Sugar alcohols, the sweeteners ending in "-ol" (sorbitol, maltitol, xylitol). They're in sugar-free gum and sweets, and in many protein bars, so check the ingredients; see GLP-1 friendly food labels.
• Lots of simple sugars, such as fructose, and milk products containing lactose if you don't digest it well.

Foods to limit on a GLP-1 explains why fat and sugar hit harder on these medicines.

Keep up your fluids

Diarrhea drains water and salts, and both the Zepbound and Wegovy labels warn that dehydration from vomiting or diarrhea can harm the kidneys. Sip water or squash through the day, in small amounts if you feel sick. Watch for the signs of dehydration: feeling thirsty, dark yellow, strong-smelling pee, peeing less often than usual and feeling dizzy. See electrolytes on a GLP-1 for more.

If you take the contraceptive pill

Two things apply. The Zepbound label already advises anyone on the pill to switch to a non-pill method, or add condoms, for four weeks after starting tirzepatide (Mounjaro or Zepbound) and after each dose increase. Separately, the NHS says that if you have diarrhea for more than 24 hours on the combined pill, keep taking it but use condoms until seven days after the diarrhea stops. A pharmacist can check what applies to your pill.

What a pharmacist can offer

• Oral rehydration sachets, a powder you mix with water, which the NHS recommends for replacing lost fluids and salts.
• Loperamide (Imodium), which the NHS says can stop diarrhea for a few hours in adults. The expert consensus says it, or a probiotic, can be considered if diarrhea persists despite diet changes.

Tell the pharmacist you're on a GLP-1. Never stop or change your dose on your own; if diarrhea keeps coming back, your prescriber can review it with you.

When to get medical help

Call your doctor or NHS 111 if you have:

• diarrhea for more than seven days
• blood in your diarrhea, or bleeding from your bottom
• vomiting and can't keep fluids down
• signs of dehydration that don't improve with drinking

Get urgent help for persistent or severe stomach pain, sometimes spreading to your back, which both US labels list as a possible sign of pancreatitis, or for vomiting blood or vomit that looks like coffee grounds.

If you also feel sick or bloated, see managing nausea and bloating on a GLP-1.

General nutrition information, not medical advice. If diarrhea is making it hard to eat or drink enough, speak to your prescriber.