Diarrhea happens when too much water ends up in your stool, either because your intestines are pulling extra fluid in, pushing food through too fast, or struggling to absorb what’s already there. Most cases are acute, lasting less than a week and resolving on their own. But when loose stools stick around for four weeks or more, something deeper is usually going on.
How Your Gut Produces Diarrhea
Your intestines normally absorb the vast majority of the fluid that passes through them, turning liquid food into formed stool. Diarrhea is what happens when that process breaks down, and it can break down in several distinct ways.
In osmotic diarrhea, something you swallowed isn’t being absorbed well, and its presence in the intestine pulls water in by osmotic force. This is the mechanism behind sugar-free candy causing loose stools, or lactose intolerance flaring up after a glass of milk. Remove the trigger and the diarrhea stops.
Secretory diarrhea is different. Here, the intestinal lining is actively pumping fluid out into the gut. Cholera is the classic extreme example, but milder versions happen with certain infections, hormonal conditions, and medications. This type tends to persist even when you stop eating.
When the intestinal lining itself is damaged, as in ulcerative colitis or certain bacterial infections, the barrier breaks down. Water, mucus, proteins, and sometimes blood leak through into the gut. This is exudative diarrhea, and it often comes with visible blood or mucus in the stool.
Finally, motility problems can cause diarrhea even when absorption is working normally. If food moves through the intestine too quickly, there simply isn’t enough time to absorb the water. An overactive thyroid is one example. Paradoxically, sluggish motility can also cause diarrhea by allowing bacteria to overgrow in the small intestine, which then triggers loose stools on its own.
Infections: The Most Common Short-Term Cause
The majority of acute diarrhea cases come from infections. Viruses are the most frequent culprits, particularly norovirus and rotavirus. These spread easily through contaminated food, water, or surfaces and typically cause watery diarrhea along with nausea and sometimes vomiting. Most viral episodes resolve within a few days without treatment.
Bacterial infections from Salmonella, Campylobacter, Shigella, and certain strains of E. coli tend to produce more intense symptoms: higher fevers, cramping, and sometimes bloody stool. These usually come from undercooked meat, contaminated produce, or unsafe water. Parasites like Giardia and Cryptosporidium are less common in developed countries but worth considering after camping trips, international travel, or exposure to untreated water. Parasitic infections often drag on for weeks if untreated.
Foods and Drinks That Trigger Loose Stools
Sometimes the cause is sitting on your plate. Fructose is one of the biggest dietary offenders. It occurs naturally in fruits like apples, pears, peaches, and cherries, and is added to sodas, juice drinks, and processed foods. Many people who consume more than 40 to 80 grams of fructose per day will develop diarrhea, because the small intestine simply can’t absorb it all. The unabsorbed sugar draws water into the gut.
Artificial sweeteners like sorbitol, mannitol, and xylitol work the same way. They’re common in sugar-free gum, candy, and some medications. Lactose, the sugar in dairy products, causes problems for people who don’t produce enough of the enzyme that breaks it down. Soft cheese, milk, and ice cream are frequent triggers.
These sugars all belong to a broader category of poorly absorbed carbohydrates sometimes called FODMAPs. Other foods in this group include wheat, rye, onions, garlic, legumes (chickpeas, lentils, beans), honey, and certain nuts like pistachios and cashews. If you notice a pattern of loose stools after meals, these foods are worth tracking.
Caffeine speeds up the digestive system and can push food through before enough water is absorbed. High-fat and fried foods are harder to digest and can trigger diarrhea in some people. Spicy sauces often mask high fat content, particularly in dishes like curries or Tex-Mex food, making it hard to pinpoint what actually caused the problem. Gluten, a protein in wheat, barley, and rye, causes diarrhea in people with celiac disease or gluten sensitivity.
Medications That Cause Diarrhea
Nearly all medications list diarrhea as a possible side effect, but some are far more likely to cause it than others.
- Antibiotics are among the most common offenders. They destroy beneficial gut bacteria along with the harmful ones, allowing other species to overgrow. In some cases, this creates an opening for a bacterium called C. difficile, which can cause severe, watery, and sometimes bloody diarrhea.
- Metformin, widely prescribed for type 2 diabetes, frequently causes diarrhea, especially when first started.
- Magnesium-containing antacids draw water into the intestine, producing an osmotic laxative effect.
- NSAIDs like ibuprofen and naproxen can irritate the gut lining.
- Proton pump inhibitors used for heartburn and acid reflux occasionally cause diarrhea as well.
- Herbal teas containing senna or other natural laxatives are an often-overlooked source, as are certain vitamin and mineral supplements.
If your diarrhea started shortly after beginning a new medication or supplement, that timing is a strong clue.
Chronic Conditions Behind Lasting Diarrhea
When diarrhea persists for four weeks or longer, it’s considered chronic and usually points to an underlying condition rather than a passing infection or dietary mistake.
Irritable bowel syndrome (IBS) is one of the most common causes. The diarrhea-predominant form produces frequent loose stools, often with cramping and urgency, but without visible damage to the intestinal lining. Inflammatory bowel diseases like Crohn’s disease and ulcerative colitis, on the other hand, involve actual inflammation and tissue damage. These conditions can cause bloody diarrhea, weight loss, and fatigue.
Celiac disease triggers an immune reaction to gluten that damages the small intestine’s absorptive surface, leading to chronic diarrhea along with bloating, nutrient deficiencies, and sometimes skin rashes. Microscopic colitis causes watery diarrhea without visible inflammation on a standard colonoscopy; it’s only detected through tissue biopsies viewed under a microscope, and it disproportionately affects older adults. Small intestinal bacterial overgrowth (SIBO) occurs when bacteria that normally live in the large intestine colonize the small intestine, fermenting food before it can be properly absorbed.
How Probiotics Can Help
Probiotics, whether from supplements or fermented foods, can reduce the severity and duration of certain types of diarrhea. A large meta-analysis in The Lancet Infectious Diseases found they reduced the risk of antibiotic-associated diarrhea by 52% and cut the risk of acute diarrhea from various causes by 34%. The benefit was even more pronounced in children, with a 57% risk reduction compared to 26% in adults.
Several strains showed similar effectiveness, including Saccharomyces boulardii and Lactobacillus rhamnosus GG. If you’re starting a course of antibiotics and want to reduce your chances of developing diarrhea, starting a probiotic at the same time is a reasonable step.
Signs That Need Medical Attention
Most diarrhea clears up on its own, but certain warning signs indicate something more serious. In adults, diarrhea lasting more than two days without improvement, a fever above 102°F, bloody or black stools, severe abdominal pain, and signs of dehydration (excessive thirst, dark urine, dizziness, very little urination) all warrant a visit to a doctor. More than 10 bowel movements a day, or fluid losses that clearly outpace what you’re able to drink, can lead to dangerous dehydration.
Children dehydrate faster than adults. A child whose diarrhea hasn’t improved within 24 hours, who has no wet diaper for three hours or more, who cries without tears, or who seems unusually sleepy or irritable needs prompt medical evaluation. Sunken eyes, a sunken soft spot on an infant’s head, or skin that stays pinched when you release it are signs of significant fluid loss.