Bad diarrhea usually comes from an infection, something you ate, or a medication you’re taking. Most cases are acute, meaning they resolve on their own within a week. But severe or persistent episodes can signal something that needs attention, and understanding the likely cause helps you figure out what to do next.
Infections Are the Most Common Cause
If your diarrhea came on suddenly and hit hard, an infection is the most likely explanation. Norovirus alone accounts for roughly 58% of food-transmitted gastroenteritis cases, making it far and away the leading culprit in adults. Bacterial infections from Salmonella, Campylobacter, and Shigella are less common but tend to cause more severe symptoms, including bloody stools and high fever.
Viral diarrhea typically runs its course in one to three days. Bacterial infections can last longer and feel worse, with cramping, urgency, and sometimes mucus or blood in the stool. Parasites like Giardia and Cryptosporidium are rarer but worth considering if your symptoms drag on for more than a couple of weeks, especially after travel or exposure to untreated water.
Your Diet May Be the Problem
Food-related diarrhea doesn’t always mean food poisoning. Certain sugars and compounds are poorly absorbed by the gut, and when they sit in your intestines undigested, they pull water in and trigger loose stools. The technical term for these compounds is FODMAPs, and they’re surprisingly common in everyday foods.
Fructose is one of the biggest offenders. It’s naturally present in fruits like apples, pears, peaches, and cherries, and it’s added to sodas, juice drinks, and applesauce. People who consume more than 40 to 80 grams of fructose per day often develop diarrhea. Sugar alcohols like sorbitol, mannitol, and xylitol, found in sugar-free gum, candy, and some medications, cause the same kind of water retention in the bowel. If you’ve recently started chewing more sugar-free gum or drinking diet beverages, that could easily explain your symptoms.
Lactose intolerance is another common trigger. If your body doesn’t produce enough of the enzyme that breaks down milk sugar, dairy products will pass through your gut largely undigested, pulling water along with them. Other dietary triggers include wheat, onions, garlic, beans, caffeine, and fried or fatty foods. Fat that isn’t absorbed in the small intestine reaches the colon, where it gets broken down into fatty acids that cause the colon to secrete fluid.
Medications That Commonly Cause Diarrhea
Nearly any medication can cause diarrhea as a side effect, but certain classes do it far more often than others. Antibiotics are the most well-known offenders. They disrupt the normal balance of bacteria in your gut, allowing certain species to overgrow. In some cases, antibiotics let a bacterium called C. difficile take over, which can cause severe, watery, and sometimes bloody diarrhea that requires its own treatment.
Other medications frequently linked to diarrhea include:
- Metformin, used for diabetes
- NSAIDs like ibuprofen and naproxen
- Magnesium-containing antacids
- Proton pump inhibitors used for heartburn and acid reflux
- Chemotherapy drugs
Herbal teas containing senna or other natural laxatives can also cause diarrhea without you realizing the connection. If your symptoms started around the same time as a new medication or supplement, that timing is worth paying attention to.
Chronic Conditions That Cause Ongoing Diarrhea
Diarrhea lasting less than a week is considered acute. If it persists beyond two weeks, it’s no longer a typical stomach bug. Diarrhea lasting four weeks or more is classified as chronic and usually points to an underlying condition.
Irritable bowel syndrome (IBS) is one of the most common causes of recurring diarrhea. It causes uncomfortable or painful digestive symptoms without visible damage to the intestines. The diarrhea-predominant form often flares with stress, certain foods, or hormonal changes. Inflammatory bowel disease (IBD), which includes Crohn’s disease and ulcerative colitis, is a different and more serious condition involving chronic inflammation that damages the intestinal lining. IBD can cause bloody diarrhea, weight loss, and fatigue.
Celiac disease is another possibility. It’s an autoimmune reaction to gluten, a protein in wheat, barley, and rye, that damages the small intestine and interferes with nutrient absorption. All three of these conditions share overlapping symptoms, and it’s possible to have more than one at the same time, which is why persistent diarrhea usually requires testing to sort out.
How to Manage It at Home
For most acute diarrhea, the priority is staying hydrated. Your body loses significant amounts of water and electrolytes with each loose stool. Signs of dehydration include extreme thirst, dark urine, urinating less than usual, dizziness, and fatigue. In young children, watch for no tears when crying, no wet diapers for three or more hours, and unusual drowsiness.
Over-the-counter options like bismuth subsalicylate (the active ingredient in Pepto-Bismol) can help reduce the frequency of loose stools. Adults can take up to 16 tablets or 16 tablespoons of regular-strength liquid in 24 hours. Avoid it if you have a bleeding disorder, kidney disease, or a stomach ulcer, and don’t combine it with aspirin or other salicylate-containing products. Loperamide (Imodium) is another option that slows intestinal movement, but skip it if you have a fever or bloody stools, as those suggest an invasive infection where slowing the gut down can make things worse.
Signs Your Diarrhea Needs Medical Attention
Most diarrhea, even when it feels terrible, resolves without medical care. But certain symptoms change the picture. Black or tarry stools, blood or pus in the stool, high fever, and signs of dehydration all warrant prompt medical evaluation. For infants, any fever alongside diarrhea is a reason to contact a doctor.
Duration matters too. If you’re past the two-week mark with no improvement, something beyond a simple infection is likely going on. Doctors can use stool tests to check for infections, parasites, and markers of intestinal inflammation. The presence of white blood cells in stool, for example, strongly suggests colitis, which could be infectious, allergic, or related to inflammatory bowel disease. Blood tests can reveal signs of nutrient malabsorption or protein loss from an inflamed gut. These tests help narrow down whether you’re dealing with something self-limiting or a condition that needs targeted treatment.