Diarrhea happens when your intestines push food through too quickly for water to be absorbed, or when something triggers your gut to release extra fluid into the stool. The cause depends on how suddenly it started, how long it’s lasted, and what else is going on in your life right now. Most cases are short-lived and triggered by a virus, something you ate, or a medication, but persistent diarrhea can point to a food intolerance or a digestive condition worth investigating.
Infections: The Most Common Cause
Viruses are the leading cause of acute diarrhea in both adults and children. Norovirus, rotavirus, and adenoviruses are the primary culprits. Norovirus symptoms typically last one to three days. Rotavirus runs a bit longer, usually five to seven days. Adenovirus infections can drag on for up to two weeks. Symptoms generally appear 24 to 60 hours after exposure, so think back a day or two to figure out where you picked it up.
Bacterial infections from contaminated food tend to hit faster. Toxins from staph bacteria can cause symptoms within 30 minutes to 8 hours of eating. Other bacteria like Salmonella typically cause symptoms within 8 to 72 hours, while Campylobacter and certain strains of E. coli take about three days. If multiple people who ate the same meal are sick, food poisoning is the likely answer.
Parasites like Giardia are less common but worth considering if you’ve been traveling, camping, or drinking untreated water.
Something You Ate or Drank
Certain sugars and sugar substitutes pull water into your intestines, creating loose stools even without an infection. Fructose is one of the biggest offenders. It’s found naturally in fruits like peaches, pears, cherries, and apples, and it’s added to sodas, juice drinks, and applesauce. Artificial sweeteners like sorbitol, mannitol, and xylitol, common in sugar-free gum, candy, and some medications, do the same thing.
Lactose in dairy products is another frequent trigger. If you notice diarrhea after milk, soft cheese, or ice cream, you may have trouble digesting lactose. These poorly absorbed sugars belong to a broader group called FODMAPs, which also includes compounds found in wheat, onions, garlic, legumes, honey, and certain nuts like pistachios and cashews. If your diarrhea keeps coming back and lines up with specific foods, a FODMAP sensitivity is worth exploring.
Medications That Cause Diarrhea
If you recently started a new medication, that may be your answer. Antibiotics are well-known offenders because they disrupt the balance of bacteria in your gut. In some cases, antibiotic use can allow a bacterium called C. difficile to overgrow, causing diarrhea that begins during or within two weeks of finishing a course of antibiotics.
Other common medications that cause diarrhea include:
- Metformin, used to treat diabetes
- NSAIDs like ibuprofen and naproxen
- Antacids containing magnesium
- Heartburn and acid reflux medications, including proton pump inhibitors like omeprazole and famotidine
- Chemotherapy drugs
- Immune-suppressing medications
If you suspect a medication, don’t stop taking it on your own. Talk to your prescriber about alternatives or ways to manage the side effect.
Chronic Diarrhea: When It Keeps Happening
Diarrhea that lasts weeks or keeps returning points to something beyond a simple infection. The most common chronic causes are irritable bowel syndrome (IBS), inflammatory bowel disease (IBD), and celiac disease.
IBS with diarrhea is diagnosed when you have recurrent abdominal pain at least one day per week, linked to bowel movements and changes in stool frequency or appearance. These symptoms need to be present for at least three months, with onset at least six months before diagnosis. There’s no single test for IBS. It’s identified based on your symptom pattern after other conditions have been ruled out.
IBD, which includes Crohn’s disease and ulcerative colitis, involves actual inflammation in the digestive tract. It often comes with blood in the stool, weight loss, and fatigue alongside diarrhea. Celiac disease is an immune reaction to gluten that damages the lining of the small intestine, leading to poor nutrient absorption and chronic loose stools. Both can be screened for with blood tests.
How Your Gut Produces Diarrhea
Not all diarrhea works the same way. In osmotic diarrhea, something in your gut (like lactose or a sugar alcohol) draws water into the intestine because your body can’t absorb it. This type tends to improve when you stop eating the offending food or fast for a period.
Secretory diarrhea is different. Toxins from bacteria or, less commonly, certain tumors cause your intestinal lining to actively pump water and salts into the bowel. This type continues regardless of whether you eat or not, which is one way doctors distinguish between the two. Infections that cause visible inflammation, blood, or mucus in the stool represent a third pattern, where the intestinal wall itself is damaged and leaking.
Staying Hydrated While You Recover
The biggest immediate risk from diarrhea is dehydration, especially if you’re also vomiting. Your body loses water, sodium, and potassium with every loose stool. Oral rehydration solutions are the most effective way to replace what you’re losing because they contain a specific balance of sugar and salts that helps your intestines absorb water efficiently.
You can buy premade rehydration solutions at most pharmacies. Sports drinks are a second-best option since they contain electrolytes but aren’t optimally formulated for rehydration during illness. Plain water alone won’t replace lost electrolytes.
The old advice to follow a strict BRAT diet (bananas, rice, applesauce, toast) is no longer recommended. The American Academy of Pediatrics considers it too restrictive, and following it for more than 24 hours may actually slow recovery because it lacks essential nutrients. Instead, eat bland, soft foods as tolerated and return to your normal diet as soon as you feel up to it.
Signs That Need Prompt Medical Attention
Most diarrhea resolves on its own, but certain symptoms call for quick action. In adults, contact a doctor if diarrhea lasts more than two days, if you’re having six or more loose stools per day, or if you develop a high fever. Black, tarry stools or stools containing red blood or pus always warrant medical evaluation. Severe abdominal or rectal pain is another signal not to wait.
Watch for signs of dehydration: extreme thirst, dry mouth, dark urine, dizziness, fatigue, or urinating much less than usual. A simple check is to pinch the skin on the back of your hand. If it doesn’t flatten back to normal right away, you may be significantly dehydrated. In infants, no wet diapers for three or more hours, no tears when crying, or a sunken soft spot on the skull are urgent warning signs. Any child under 12 months with diarrhea should be seen by a doctor promptly.