Diarrhea happens when too much water ends up in your stool, either because your intestines are pulling extra fluid in or because they’ve lost the ability to absorb it properly. The cause is usually something straightforward: a virus you picked up, something you ate, a medication you’re taking, or stress. Figuring out which one depends on when it started, how long it’s lasted, and what else is going on in your body.
How Diarrhea Actually Works in Your Gut
Your intestines handle an enormous amount of fluid every day, absorbing most of it back into your body. Diarrhea disrupts that process in one of two ways. In the first, something in your gut pulls water into the intestinal space through osmotic pressure. Think of it like a sponge effect: unabsorbed sugars, sugar alcohols, or other small molecules draw water toward them, flooding the space faster than your colon can reabsorb it. This is why sugar-free candy (loaded with sugar alcohols) can send you running to the bathroom.
In the second type, your intestinal lining is actively pumping fluid out. Certain infections and toxins hijack the cells lining your gut, forcing them to secrete chloride and water into the intestinal space while simultaneously blocking sodium absorption. Cholera is the extreme example, but milder bacterial toxins do the same thing on a smaller scale. Both mechanisms produce loose, watery stool, but they get there by different routes.
Viral and Bacterial Infections
Infections are the single most common cause of sudden diarrhea. Norovirus is the usual culprit in adults, with symptoms hitting 12 to 48 hours after exposure and typically resolving within 12 to 60 hours. It spreads easily through contaminated food, surfaces, or close contact with someone who’s sick. If multiple people who shared a meal all get sick within the same 24-hour window, norovirus is a strong bet.
Bacterial food poisoning from organisms like Salmonella tends to show up 12 to 36 hours after eating contaminated food, often undercooked poultry, eggs, or unwashed produce. The diarrhea is frequently more severe than viral illness and may include fever or bloody stool.
Parasitic infections like Giardia take longer to announce themselves. The incubation period is 7 to 10 days, and symptoms can persist for one to two weeks. Giardia is commonly picked up from contaminated water, especially while camping, traveling, or swimming in untreated freshwater. If your diarrhea started a week or more after a trip and won’t go away, a parasitic cause is worth considering.
Foods That Trigger Loose Stool
Certain foods cause diarrhea not because they’re spoiled, but because your body can’t fully absorb them. Fructose is a prime example. Your small intestine has a limited capacity to absorb fructose, relying on a specific transport protein that can be overwhelmed. When excess fructose reaches the colon unabsorbed, it pulls water into the intestinal space and gets fermented by bacteria, producing gas and loose stool. Apples, pears, mangoes, and Asian pears contain more excess free fructose per serving than most other fruits, making them common triggers.
Lactose intolerance works the same way. Without enough of the enzyme that breaks down milk sugar, lactose passes into the colon intact, draws water in, and gets fermented. The result is bloating, cramping, and diarrhea within a few hours of eating dairy.
Fructans, a type of carbohydrate found in wheat, onions, and garlic, can also cause trouble, though studies show symptoms generally only appear at doses above 20 grams in a single sitting. That’s a fairly large amount, but if you ate a big pasta dinner loaded with garlic bread and onion-heavy sauce, you could hit that threshold. Spicy foods, alcohol, caffeine, and high-fat meals round out the list of common dietary triggers.
Medications, Especially Antibiotics
Antibiotic-associated diarrhea affects up to 30% of people taking antibiotics. These drugs don’t just kill the bacteria causing your infection. They reduce the diversity of your entire gut microbiome, wiping out beneficial species that normally help you digest carbohydrates and bile acids. Without those microbes, carbohydrate and bile metabolism get disrupted, creating an osmotic imbalance that draws water into the colon. The loss of protective bacteria also opens the door for harmful organisms to overgrow, which can make the diarrhea worse or longer-lasting.
Antibiotics aren’t the only medication that does this. Antacids containing magnesium, certain blood pressure medications, and diabetes drugs are also frequent offenders. If your diarrhea started within a few days of beginning a new medication, the connection is likely not a coincidence.
Stress and Anxiety
Your gut and brain communicate constantly through a network of nerves, and emotional stress can speed up intestinal contractions. When food moves through your colon too quickly, your body doesn’t have enough time to reabsorb water, and the result is loose stool. This is why diarrhea before a job interview, an exam, or a flight is so common. If your episodes consistently line up with stressful events and resolve once the stress passes, that’s a strong clue.
When Diarrhea Keeps Coming Back
Occasional diarrhea is normal. Recurring episodes that follow a pattern point to something chronic. Irritable bowel syndrome (IBS) is the most common explanation: it’s defined by at least 12 weeks of abdominal discomfort over a 12-month period, combined with changes in stool frequency or consistency that improve after a bowel movement. IBS doesn’t cause visible inflammation or damage to the intestines. Diagnostic imaging looks completely normal, and there’s no increased risk of colon cancer.
Inflammatory bowel disease (IBD), which includes Crohn’s disease and ulcerative colitis, is a different situation entirely. IBD causes real, measurable inflammation that can be seen on scans and during colonoscopy. It can permanently damage the intestinal lining and raises the risk of colon cancer. Key warning signs that point toward IBD rather than IBS include anemia, blood in the stool, unexplained weight loss, and fever. These symptoms don’t occur in IBS.
What to Eat During and After an Episode
The old advice to stick strictly to bananas, rice, applesauce, and toast has largely fallen out of favor. The National Institute of Diabetes and Digestive and Kidney Diseases notes that most experts no longer recommend following a restricted diet or fasting during acute diarrhea. Once you feel like eating, your normal diet is fine. Children should continue their usual age-appropriate foods, and infants should keep getting breast milk or formula.
The priority is fluid replacement. Diarrhea pulls water and electrolytes out of your body quickly. Small, frequent sips of water, broth, or an oral rehydration solution work better than drinking large amounts at once. Avoid alcohol, caffeine, and very sugary drinks, which can pull more water into the gut and make things worse.
Signs That Need Medical Attention
Most diarrhea resolves on its own within a day or two. Certain symptoms signal something more serious. In adults, these include diarrhea lasting more than two days without improvement, a fever above 102°F (39°C), bloody or black stools, severe abdominal or rectal pain, and signs of dehydration like excessive thirst, very dark urine, dizziness, or skin that stays pinched when you pull it up and release it. More than 10 bowel movements a day, or losing fluid faster than you can drink it, qualifies as severe.
For children, the timeline is shorter. Diarrhea that hasn’t improved within 24 hours, no wet diaper in three or more hours, a fever above 102°F, or blood in the stool all warrant prompt evaluation. Young children and older adults dehydrate faster and have less margin for error.