Diarrhea has dozens of possible causes, ranging from a virus that clears up in a day or two to chronic digestive conditions that persist for months. The cause usually depends on how long symptoms last: episodes shorter than two weeks are almost always triggered by infections or something you ate, while diarrhea lasting beyond four weeks points to an underlying medical condition, a medication side effect, or a food intolerance.
How Duration Helps Narrow the Cause
Gastroenterologists classify diarrhea into three categories based on timing. Acute diarrhea lasts less than two weeks and is by far the most common type. Persistent diarrhea runs from two to four weeks. Chronic diarrhea continues beyond four weeks. These timeframes matter because each window is associated with a different set of causes, and knowing where you fall helps determine whether the problem will resolve on its own or needs investigation.
Infections: The Most Common Short-Term Cause
Viruses are the leading cause of acute diarrhea worldwide. Rotavirus and norovirus account for the largest share of cases, with rotavirus alone responsible for roughly a third of diarrhea-related hospitalizations in children. These infections spread through contaminated hands, surfaces, and food, and they typically resolve within a few days without specific treatment beyond staying hydrated.
Bacterial infections tend to produce more severe symptoms, often with fever, cramping, and sometimes blood in the stool. The most common culprits include several strains of E. coli, Salmonella, Campylobacter, and Shigella. Contaminated or undercooked food is a frequent source, particularly ground beef, poultry, unpasteurized dairy, and produce washed in unsafe water. Traveler’s diarrhea is typically caused by bacteria encountered in regions with different water and sanitation standards than what your gut is accustomed to.
Parasites like Giardia and Cryptosporidium cause diarrhea that can drag on for weeks if untreated. These organisms are often picked up through contaminated water sources, including streams and lakes that look clean, and through person-to-person contact in daycare or institutional settings.
Medications That Trigger Diarrhea
Nearly any medication can cause diarrhea, but certain classes do it far more often than others. Antibiotics are among the most common offenders. They disrupt the balance of bacteria in your gut, which can cause loose stools on its own or, in more serious cases, allow a harmful bacterium called C. difficile to take over. C. difficile infections can cause severe, watery diarrhea that becomes chronic if not treated.
Other medications frequently linked to diarrhea include:
- Magnesium-containing antacids, which draw water into the intestines
- Proton pump inhibitors and acid reducers used for heartburn and ulcers
- Metformin, a widely used diabetes medication
- NSAIDs like ibuprofen and naproxen
- Chemotherapy drugs
- Immune-suppressing medications
Laxatives, of course, are designed to cause this effect. Some herbal teas and supplements contain senna or other natural laxatives that can produce diarrhea without the user realizing why.
Food Intolerances and Dietary Triggers
Lactose intolerance is one of the most widespread dietary causes of diarrhea. When your body doesn’t produce enough of the enzyme that breaks down lactose (the sugar in milk), the undigested sugar pulls water into the intestines and ferments, causing bloating, gas, and loose stools. A similar process happens with fructose, the sugar found in fruit, honey, and high-fructose corn syrup, when consumed in amounts that exceed your gut’s ability to absorb it.
Sugar alcohols are another common and often overlooked trigger. These are the low-calorie sweeteners found in sugar-free gum, candy, protein bars, and many “diet” products. Your body can’t fully digest them, so they linger in the intestines, draw in water, and ferment. Xylitol and sorbitol are particularly likely to cause bloating, gas, and diarrhea. The FDA actually requires products containing sorbitol or mannitol to carry a warning that excessive consumption can have a laxative effect. People with IBS or Crohn’s disease are especially sensitive to these sweeteners, but anyone who eats enough of them will feel the effects.
Gluten triggers diarrhea in people with celiac disease, an autoimmune condition in which gluten damages the lining of the small intestine. This is different from a simple food sensitivity. Celiac disease causes chronic diarrhea along with nutrient malabsorption and, over time, weight loss and fatigue.
Chronic Digestive Conditions
When diarrhea persists for more than four weeks, an underlying digestive disorder is often responsible. The most common include:
- Irritable bowel syndrome (IBS), a gut-brain disorder that causes recurring abdominal pain along with diarrhea, constipation, or both. It’s one of the most frequent causes of chronic diarrhea and is driven by how the brain and digestive system communicate rather than by visible damage to the intestines.
- Crohn’s disease and ulcerative colitis, two forms of inflammatory bowel disease in which the immune system attacks the intestinal lining. Both cause diarrhea that may contain blood or mucus, along with cramping and fatigue.
- Celiac disease, triggered by gluten in wheat, barley, and rye.
- Small intestinal bacterial overgrowth (SIBO), in which excess bacteria in the small intestine interfere with digestion and nutrient absorption.
- Pancreatic insufficiency, where the pancreas doesn’t produce enough digestive enzymes, leading to fatty, foul-smelling stools.
Diabetes can also cause chronic diarrhea through nerve damage (autonomic neuropathy) that disrupts the normal movement of food through the intestines. This tends to develop in people who have had poorly controlled blood sugar for years.
Surgery as a Cause
Certain abdominal surgeries can leave you with chronic diarrhea, sometimes permanently. Removal of the gallbladder is a well-known example: without the gallbladder to regulate bile, excess bile acids enter the large intestine and stimulate fluid secretion. Surgery on the stomach, esophagus, pancreas, liver, or portions of the small or large intestine can all alter how your body digests and absorbs food. Stomach or esophageal surgery sometimes leads to a condition called dumping syndrome, where food moves too quickly into the small intestine, causing diarrhea, cramping, and lightheadedness after meals.
How the Gut Produces Diarrhea
Regardless of the trigger, diarrhea happens through a few basic mechanisms in the intestines. In osmotic diarrhea, something in the gut (undigested lactose, a sugar alcohol, a poorly absorbed nutrient) pulls water into the intestines by osmosis. This type usually improves when you stop eating the offending substance.
Secretory diarrhea occurs when the intestinal lining actively pumps out more fluid than it absorbs. Bacterial toxins, certain hormones, and bile acids can all flip this switch. The hallmark of secretory diarrhea is that it tends to be watery, high-volume, and doesn’t improve with fasting.
In inflammatory or exudative diarrhea, the intestinal lining itself is damaged. When the barrier breaks down, water, electrolytes, mucus, and proteins leak into the intestine. This is what happens in ulcerative colitis, Crohn’s disease, and infections like Shigella. If this continues long enough, the ongoing protein loss can deplete your blood protein levels and cause additional symptoms like swelling.
Signs That Point to a Serious Cause
Most acute diarrhea resolves on its own. But certain patterns suggest something that needs medical attention. Blood or pus in the stool can indicate a bacterial infection or inflammatory bowel disease. Diarrhea accompanied by a high fever, severe abdominal pain, or signs of dehydration (dark urine, dizziness, rapid heartbeat) warrants prompt evaluation. Unintentional weight loss alongside chronic diarrhea is a signal that your body isn’t absorbing nutrients properly, which can point to celiac disease, Crohn’s disease, or pancreatic problems. And diarrhea that begins during or shortly after a course of antibiotics should raise concern for a C. difficile infection, particularly in older adults.