Diarrhea can be a sign of dozens of different things, from a simple stomach bug to a chronic digestive condition. Most cases are acute, meaning they last one or two days and resolve on their own, typically caused by a virus or something you ate. But diarrhea lasting four weeks or more can point to an underlying condition that needs attention. The cause depends largely on how long it lasts, what it looks like, and what other symptoms come with it.
Viral and Bacterial Infections
The most common cause of short-term diarrhea is infection. Viruses are the biggest culprit, with rotavirus and norovirus leading the pack. These spread easily through contaminated food, water, or close contact with someone who’s sick, and they typically cause watery diarrhea along with nausea, vomiting, and sometimes a low fever. Most viral cases clear up within a few days without treatment.
Bacterial infections tend to hit harder. Salmonella, Campylobacter, Shigella, and certain strains of E. coli can all cause diarrhea that’s more severe, sometimes bloody, and often accompanied by cramping and higher fevers. These are usually picked up from undercooked meat, contaminated produce, or unsafe water. Parasites like Giardia and Cryptosporidium are less common but can cause diarrhea that drags on for weeks if untreated, especially after drinking untreated water while camping or traveling abroad.
Food Intolerances and Malabsorption
If diarrhea keeps coming back after eating certain foods, a food intolerance is a likely explanation. Lactose intolerance is the most common example. People who lack the enzyme to break down lactose (the sugar in milk) experience bloating, gas, and loose stools after consuming dairy. Fructose malabsorption works similarly and is surprisingly common.
Sugar alcohols like sorbitol, mannitol, and xylitol, found in sugar-free gum, candy, and some protein bars, are another frequent trigger. These sweeteners aren’t fully absorbed in the small intestine, so they pull water into the gut and get fermented by bacteria in the colon, producing gas and watery stools.
Fat malabsorption is a different pattern. When fats aren’t properly absorbed in the small intestine, they pass into the colon and produce greasy, foul-smelling stools that may be light-colored and float. This can result from conditions affecting the pancreas, liver, or bile ducts. Bile acid malabsorption, where leftover bile salts reach the colon, triggers the colon to secrete water, leading to chronic watery diarrhea.
Chronic Digestive Conditions
Diarrhea lasting four weeks or longer often signals an underlying digestive disease. The most common ones include:
- Irritable bowel syndrome (IBS): Causes recurring episodes of diarrhea (sometimes alternating with constipation), abdominal pain, and bloating. It’s a disorder of gut-brain communication rather than visible damage to the intestine.
- Celiac disease: An immune reaction to gluten that damages the lining of the small intestine, leading to malabsorption. Diarrhea, weight loss, fatigue, and nutritional deficiencies are hallmarks.
- Crohn’s disease: A type of inflammatory bowel disease that can affect any part of the digestive tract. It causes chronic diarrhea, abdominal pain, and sometimes bloody stools.
- Ulcerative colitis: Another inflammatory bowel disease, but limited to the colon. Bloody diarrhea with urgency is the signature symptom.
- Microscopic colitis: Causes chronic watery diarrhea without visible inflammation on a standard colonoscopy. It’s diagnosed through tissue biopsy and is more common in older adults.
Medications That Cause Diarrhea
Diarrhea is one of the most common medication side effects, and it’s easy to overlook the connection. Antibiotics are a well-known cause. They work by killing bacteria, but they also wipe out beneficial gut bacteria, allowing other species to overgrow. In some cases, this leads to an overgrowth of a particularly dangerous bacterium called C. difficile, which can cause severe, persistent diarrhea.
Other medications that commonly cause diarrhea include metformin (used for diabetes), NSAIDs like ibuprofen and naproxen, proton pump inhibitors used for heartburn, chemotherapy drugs, and magnesium-containing antacids. Even some herbal teas contain natural laxatives like senna that can trigger loose stools. If diarrhea started around the same time you began a new medication or supplement, that’s worth mentioning to your doctor.
Stress and the Gut-Brain Connection
Stress, anxiety, and other psychological factors can directly alter how your digestive system works. The gut and brain communicate through a dense network of nerves, and emotional distress can speed up contractions in the GI tract, pushing food through too quickly for water to be properly absorbed. This is why some people get diarrhea before a big presentation, during a stressful week, or in periods of ongoing anxiety. It’s not “in your head.” Stress physically changes the movement and contractions of the gut. For people with IBS, this connection is especially pronounced, and managing stress often improves symptoms significantly.
What the Timing and Appearance Tell You
How long diarrhea lasts is one of the most useful clues to its cause. A day or two of loose stools after eating something questionable almost always points to an infection or a food reaction. When it stretches beyond two weeks, infections like parasites, medication effects, or early signs of a chronic condition become more likely. At the four-week mark, chronic causes like IBS, inflammatory bowel disease, or celiac disease move to the top of the list.
The appearance matters too. Watery diarrhea without blood usually points to viral infections, food intolerances, or conditions that affect how water moves through the intestine. Bloody or black stools suggest inflammation or infection involving the intestinal lining, as seen in bacterial infections, ulcerative colitis, or Crohn’s disease. Pale, greasy, floating stools that smell especially bad point toward fat malabsorption.
Dehydration: The Main Immediate Risk
Regardless of the cause, the biggest short-term danger of diarrhea is dehydration. In adults, warning signs include dark urine, urinating less than usual, sunken eyes, and skin that doesn’t spring back quickly when pinched. In infants and young children, watch for no wet diapers for three hours, no tears when crying, a dry mouth, and a sunken soft spot on the skull. Young children and older adults are most vulnerable because their bodies have less margin for fluid loss.
For adults, diarrhea that doesn’t improve after two days, a fever above 102°F, or bloody or black stools all warrant medical attention. For children, the threshold is shorter: seek care if it hasn’t improved within 24 hours, or sooner if a high fever or bloody stools are present.