Diarrhea has dozens of possible causes, ranging from something you ate last night to a chronic condition you haven’t been diagnosed with yet. Most cases are short-lived and triggered by infections, food reactions, or medications. But persistent or recurring diarrhea often points to something deeper, like a food intolerance, a digestive disorder, or even a hormonal imbalance. Here’s a practical breakdown of the most common culpraces.
Infections: Bacteria, Viruses, and Parasites
The most frequent cause of sudden diarrhea is an infection picked up from contaminated food, water, or surfaces. Bacteria are the dominant culprit, responsible for roughly a third of acute cases. The top offenders are Salmonella (often from undercooked poultry or eggs), certain strains of E. coli (from contaminated produce or undercooked beef), and Campylobacter (commonly linked to raw or undercooked chicken). These infections typically hit within hours to a couple of days after exposure and cause watery or bloody stools, cramping, and sometimes fever.
Viruses are the other big category. Norovirus is the most common viral cause in adults, spreading easily through close contact and contaminated surfaces. It’s the classic “stomach bug” that tears through households, cruise ships, and schools. Rotavirus used to be the leading cause in young children before widespread vaccination.
Parasites like Giardia and Cryptosporidium are less common but worth knowing about, especially if your diarrhea lingers for weeks. These organisms often come from contaminated water sources, including streams and lakes, and can cause prolonged, intermittent symptoms that don’t resolve on their own without treatment.
Traveler’s Diarrhea
If you’ve ever gotten sick on a trip abroad, you’re far from alone. Between 30% and 70% of travelers develop diarrhea within two weeks of arriving in a high-risk region, depending on the destination and time of year. Bacteria cause more than 80% of these cases. E. coli strains are the leading cause across South and Central Asia, North Africa, the Middle East, sub-Saharan Africa, and Latin America. Southeast and East Asia tend to have higher rates of Campylobacter and Salmonella instead.
The usual route of infection is consuming local water or food prepared with it. Ice in drinks, washed salads, and street food are common sources. Most episodes resolve within a few days, but they can be severe enough to derail a trip.
Foods and Sugar Substitutes
Certain foods cause diarrhea not because they’re contaminated, but because your body can’t absorb them properly. When unabsorbed sugars sit in your intestines, they pull water into the gut through osmotic pressure, creating loose, watery stools.
Fructose is a major example. Your small intestine has a limited capacity to absorb it, and when you overwhelm that capacity, the excess ferments in your colon. Fruits with high amounts of excess fructose, particularly apples, pears, and mangoes, are the most likely to cause problems. Soft drinks, fruit juices, and candy sweetened with high-fructose corn syrup are also common triggers. A study of over 3,400 patients with fructose malabsorption found that 52% were consuming diets heavy in these foods.
Sugar alcohols like sorbitol and mannitol, found in “sugar-free” gums, candies, and protein bars, work the same way. As little as 10 grams of sorbitol can cause bloating and gas in most people, and 20 grams reliably triggers cramping and diarrhea. A single pack of sugar-free candy can easily contain that much. In fact, a CDC report once traced a diarrhea outbreak in New Hampshire to dietetic candies sweetened with sorbitol.
Lactose and Other Intolerances
Lactose intolerance is one of the most common reasons for recurring diarrhea after meals. If your body doesn’t produce enough of the enzyme that breaks down milk sugar, that undigested lactose travels to the colon and draws water in, producing gas, bloating, and loose stools. This affects a large percentage of adults worldwide, especially those of East Asian, African, and Native American descent.
The tricky part is that lactose hides in foods you might not suspect: bread, salad dressings, processed meats, and many medications use lactose as a filler. If dairy seems to be the pattern, try removing it for a couple of weeks and see if your symptoms clear up.
Medications, Especially Antibiotics
About 1 in 5 people who take antibiotics develop diarrhea as a side effect. Antibiotics don’t just kill the bacteria making you sick. They also wipe out beneficial gut bacteria that help regulate digestion. Without that balance, harmful bacteria can multiply unchecked. The most dangerous of these is C. difficile, which can cause severe, sometimes life-threatening diarrhea, particularly in older adults or people who’ve had repeated antibiotic courses.
Antibiotics aren’t the only medications that cause problems. Magnesium supplements, especially in citrate form, have a well-known laxative effect. That’s why magnesium citrate is actually sold as a laxative at pharmacies. If you’re taking magnesium for muscle cramps or sleep and noticing loose stools, the supplement is very likely the cause. Other common offenders include heartburn medications (proton pump inhibitors), the diabetes drug metformin, and certain blood pressure medications.
Chronic Digestive Conditions
When diarrhea keeps coming back over weeks or months, it often signals an underlying condition rather than a one-time trigger.
Irritable bowel syndrome with diarrhea (IBS-D) is one of the most common diagnoses. IBS does not cause inflammation or physical damage to the digestive tract, which is the critical distinction between it and more serious conditions. It’s a disorder of gut-brain communication that causes cramping, bloating, and alternating bowel habits. Stress, certain foods, and hormonal changes can all set it off.
Inflammatory bowel disease, which includes Crohn’s disease and ulcerative colitis, is fundamentally different. IBD is an autoimmune condition that causes actual damage to the lining of the digestive tract. It can produce bloody diarrhea, weight loss, fatigue, and symptoms outside the gut like joint pain and skin problems. Unlike IBS, IBD typically shows up on imaging or during a colonoscopy.
Celiac disease is another chronic cause. It’s triggered specifically by gluten, a protein in wheat, barley, and rye. In people with celiac disease, eating gluten damages the lining of the small intestine, impairing nutrient absorption and causing diarrhea, bloating, and fatigue. The damage is confined to the small intestine, unlike IBD, which can affect any part of the digestive system.
Bile Acid Malabsorption
Your liver produces bile acids to help digest fats. Normally, your small intestine reabsorbs most of these acids at the end of digestion. But when that reabsorption fails, excess bile acids spill into the colon, where they trigger the colon to secrete extra water, producing urgent, watery diarrhea. This is called bile acid malabsorption, and it’s an underdiagnosed cause of chronic loose stools.
It commonly develops after gallbladder removal surgery, because without a gallbladder to regulate bile release, a steady stream of bile flows directly into the intestines. It can also result from damage to the small intestine from Crohn’s disease, radiation therapy, or other surgeries. If your diarrhea started after having your gallbladder out and never fully resolved, bile acid malabsorption is a likely explanation worth raising with your doctor.
Hormonal Causes
An overactive thyroid gland speeds up nearly every system in the body, including digestion. Hyperthyroidism increases gut motility, meaning food moves through the intestines faster than normal, leaving less time for water to be absorbed. The result is more frequent bowel movements and, in more severe cases, persistent diarrhea. Other signs include unexplained weight loss, rapid heartbeat, anxiety, and heat intolerance. If your diarrhea comes alongside these symptoms, a simple blood test can check your thyroid levels.
Warning Signs That Need Attention
Most diarrhea resolves on its own within a day or two. But certain symptoms suggest something more serious is going on. In adults, diarrhea lasting more than two days, six or more loose stools per day, high fever, severe abdominal pain, or stools that are black, tarry, or contain blood or pus all warrant prompt medical evaluation.
Dehydration is the most immediate risk, particularly for young children and older adults. Watch for extreme thirst, dark urine, urinating much less than usual, dizziness, and fatigue. In infants, no wet diapers for three or more hours, no tears when crying, or a sunken soft spot on the skull are signs of significant dehydration that need urgent care. For any child under 12 months, even a single day of diarrhea is worth a call to the pediatrician.