Diarrhea has dozens of possible causes, ranging from a virus that clears in a day to a chronic digestive condition that needs long-term management. The cause matters because it determines whether you wait it out, change what you’re eating, or get medical help. Loose or watery stools happening three or more times in 24 hours counts as diarrhea, and if it lasts four weeks or longer, it’s considered chronic.
Infections: The Most Common Short-Term Cause
Most sudden-onset diarrhea is caused by a virus, bacterium, or parasite picked up from contaminated food, water, or close contact with someone who’s sick. Norovirus is the single most common viral culprit, spreading easily in households, cruise ships, and restaurants. Rotavirus, once a leading cause in young children, has become less common in countries with routine vaccination but still circulates worldwide. Adenovirus and sapovirus round out the viral list.
Bacterial infections tend to cause more severe symptoms. Salmonella, certain strains of E. coli, and Campylobacter are the bacteria most frequently identified in cases of acute infectious diarrhea. These typically come from undercooked poultry, contaminated produce, or improperly stored food. Symptoms often include cramping, fever, and sometimes blood in the stool, and they can take longer to resolve than a typical stomach virus.
Parasites like Giardia and Cryptosporidium are less common in developed countries but show up after exposure to untreated water from lakes or streams, or during outbreaks linked to swimming pools and water parks. Parasitic diarrhea tends to be more persistent, sometimes lasting weeks without treatment.
Traveler’s Diarrhea
If you’ve ever gotten sick on a trip abroad, bacteria were almost certainly responsible. The CDC estimates that bacteria account for 75% to 90% of traveler’s diarrhea cases. Enterotoxigenic E. coli is the pathogen identified most often, followed by Campylobacter, Shigella, and Salmonella. Poor hygiene practices in local restaurants and gaps in sanitation infrastructure are the main drivers, and these factors are largely outside your control as a traveler. Even in high-income countries, restaurant food handling has been linked to outbreaks of bacterial diarrhea.
Foods and Drinks That Trigger It Directly
Some foods cause diarrhea not through infection but through their chemical makeup. When your gut can’t absorb a particular sugar or sugar alcohol, water gets pulled into the intestine to dilute it. This is called osmotic diarrhea, and it stops when you stop eating the trigger.
Lactose intolerance is probably the most familiar example. If your body doesn’t produce enough of the enzyme that breaks down milk sugar, dairy products send water rushing into your bowel. Fructose, the sugar found in fruit and honey, can do the same thing when consumed in large amounts, especially in concentrated forms like fruit juice or high-fructose corn syrup.
Sugar alcohols are a surprisingly common and frequently overlooked cause. Sorbitol, mannitol, xylitol, maltitol, and erythritol all have known laxative effects. Sorbitol appears naturally in apples, pears, peaches, plums, prunes, and several other fruits, and it’s added to sugar-free gum, candy, and even some liquid medications. As little as 5 to 20 grams per day can cause gas, bloating, urgency, and cramping. Doses above 20 grams reliably cause diarrhea. To put that in perspective, a single pack of Trident gum contains 20 to 22.5 grams of sorbitol. Chewing through a pack over the course of a day could easily push you past that threshold without you ever suspecting the gum.
Caffeine and alcohol also speed up gut motility. Coffee stimulates contractions in the colon, and alcohol, especially in excess, can irritate the intestinal lining and impair absorption.
Medications
Drug-induced diarrhea is more common than most people realize, and antibiotics are the biggest offenders. Antibiotics kill harmful bacteria, but they also wipe out beneficial gut bacteria, disrupting the balance in your intestine. This can cause loose stools during or shortly after a course of treatment. In some cases, the disruption allows a bacterium called C. difficile to overgrow, causing more severe and persistent diarrhea.
Magnesium-containing antacids are another frequent cause. Magnesium draws water into the intestine in the same way sugar alcohols do. Metformin, one of the most widely prescribed diabetes medications, commonly causes diarrhea as a side effect, particularly when starting the drug or increasing the dose. Other medications linked to diarrhea include certain blood pressure drugs, chemotherapy agents, and nonsteroidal anti-inflammatory drugs like ibuprofen.
Chronic Digestive Conditions
When diarrhea persists for four weeks or more, it’s likely driven by something other than an infection. Irritable bowel syndrome (IBS) is the most common cause of chronic diarrhea in developed countries, with an estimated prevalence of 4% to 9% in the United States. The diarrhea-predominant subtype, known as IBS-D, affects up to 40% of adults diagnosed with IBS. IBS doesn’t damage the intestine, but it disrupts the communication between the brain and the gut, leading to cramping, urgency, and unpredictable bowel habits that can be triggered by stress, certain foods, or hormonal changes.
Inflammatory bowel diseases, specifically Crohn’s disease and ulcerative colitis, cause diarrhea through actual inflammation and damage to the intestinal lining. These conditions often involve blood in the stool, weight loss, fatigue, and periods of flare and remission. Celiac disease, an autoimmune reaction to gluten, damages the lining of the small intestine and impairs nutrient absorption, producing diarrhea along with bloating, fatigue, and sometimes anemia or bone loss.
Less commonly recognized causes include exocrine pancreatic insufficiency (EPI), where the pancreas doesn’t produce enough digestive enzymes to properly break down food. Studies estimate that 5% to 6% of patients who meet the diagnostic criteria for IBS-D actually have undetected EPI. Hyperthyroidism, an overactive thyroid gland, can also speed up the digestive tract enough to cause chronic loose stools.
How Different Types Feel Different
One useful distinction is between osmotic and secretory diarrhea, because they behave differently and point to different causes. Osmotic diarrhea is driven by something in your gut pulling water in, whether that’s lactose, sorbitol, or another poorly absorbed substance. The hallmark: it stops when you stop eating. If your diarrhea resolves when you fast or eliminate a specific food, an osmotic trigger is likely.
Secretory diarrhea happens when the intestinal lining actively pumps extra fluid into the bowel. It continues regardless of whether you eat. Certain bacterial toxins, hormonal imbalances, and rare tumors can drive this type. If diarrhea persists even when you haven’t eaten for several hours, that’s a clue it may be secretory and worth investigating with a doctor.
Warning Signs That Need Prompt Attention
Most diarrhea resolves on its own within a few days. But certain symptoms signal something more serious. In adults, seek medical care if diarrhea lasts more than two days without improvement, if you develop a fever above 102°F, if stools are bloody or black, or if you notice signs of dehydration: excessive thirst, dark urine, dizziness, dry mouth, or very little urination.
Children need closer monitoring. A child whose diarrhea hasn’t improved in 24 hours, who hasn’t had a wet diaper in three or more hours, or who cries without tears needs medical evaluation. Sunken eyes, a sunken soft spot on an infant’s head, unusual sleepiness, and skin that stays pinched instead of flattening back are signs of significant dehydration. Severe diarrhea, defined as more than 10 bowel movements a day or fluid losses that outpace what you can drink, can become life-threatening without treatment.