Diarrhea happens when the normal balance between fluid absorption and fluid secretion in your intestines gets disrupted. Something, whether an infection, a food, or a medication, causes your gut to either pull extra water into the intestinal space or fail to absorb the water that’s already there. The result is loose, watery stools that move through your system faster than usual. The causes range from a bad meal to a chronic digestive condition, and figuring out which one applies to you depends largely on how long it lasts and what other symptoms show up alongside it.
Infections: The Most Common Cause
Most episodes of acute diarrhea are caused by infections picked up through contaminated food, contaminated water, or direct person-to-person contact due to poor hand hygiene. The culprits fall into three categories: viruses, bacteria, and parasites.
Viruses are responsible for the majority of short-lived diarrhea in otherwise healthy people. Norovirus is the leading cause in adults, while rotavirus historically hit young children hardest (though vaccination has dramatically reduced those cases). These viruses spread easily in close quarters, which is why outbreaks rip through cruise ships, daycare centers, and nursing homes.
Bacterial infections tend to cause more severe episodes. E. coli, Salmonella, and Shigella are among the most common offenders, typically picked up from undercooked meat, contaminated produce, or improperly stored food. Some bacterial infections cause watery diarrhea that resolves in a few days. Others cause what’s known as dysentery, where stools contain blood or mucus, signaling deeper damage to the intestinal lining.
Parasites like Giardia and Cryptosporidium are less common in developed countries but still show up, particularly through contaminated water sources. Parasitic infections tend to linger longer than viral or bacterial ones.
How Your Gut Actually Produces Diarrhea
Your intestines process roughly 9 liters of fluid every day, most of which gets reabsorbed back into your body. Diarrhea happens when that reabsorption process breaks down, and it does so in two distinct ways.
In osmotic diarrhea, something sitting in your intestines draws water in. Undigested sugars are the classic example. When your body can’t break down lactose or fructose, those molecules pull fluid into the gut through osmosis. This type of diarrhea stops when you stop eating the trigger, which is why it goes away during fasting.
In secretory diarrhea, your intestinal lining actively pumps fluid outward, often because a bacterial toxin has hijacked the signaling pathways in your gut cells. Cholera is the extreme version of this, but milder toxin-driven diarrhea from food poisoning works the same way. The key difference: secretory diarrhea continues even if you stop eating entirely.
Foods and Drinks That Trigger It
Sugars are one of the biggest dietary triggers. They stimulate the gut to release water and electrolytes, loosening stools. Fructose is a particularly common offender. It’s found naturally in fruits like peaches, pears, cherries, and apples, 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 from it alone.
Sugar alcohols used in sugar-free products, including sorbitol, mannitol, and xylitol, are another frequent cause. These show up in sugar-free gum, candy, and some medications. Your small intestine can’t fully absorb them, so they travel to the large intestine where they pull in water and get fermented by bacteria, producing gas and loose stools. Fructose, lactose, and these sugar alcohols all belong to a group of poorly digested carbohydrates sometimes called FODMAPs, which are a well-known trigger for people with sensitive guts.
Lactose intolerance deserves its own mention because it’s so widespread. If your body doesn’t produce enough of the enzyme that breaks down milk sugar, dairy products will send undigested lactose into your large intestine, where it causes bloating, cramping, and diarrhea.
Medications That Cause Diarrhea
Antibiotics are one of the most common medication-related causes. They kill the bacteria making you sick, but they also wipe out beneficial bacteria in your gut, disrupting the microbial balance that keeps digestion running smoothly. This imbalance can cause diarrhea on its own, and it also opens the door to more dangerous infections.
One of those dangers is C. diff, a bacteria that thrives when antibiotics clear out its competition. People are up to 10 times more likely to develop a C. diff infection while taking antibiotics and during the month after finishing a course. Longer courses of antibiotics can double the risk further. C. diff infections are most common in healthcare settings and among people over 65, those with weakened immune systems, or anyone who has had a previous C. diff infection.
Beyond antibiotics, several other medication types commonly cause diarrhea:
- Magnesium-containing antacids, which draw water into the intestines
- Heartburn and ulcer medications such as proton pump inhibitors
- NSAIDs like ibuprofen and naproxen, used for pain and inflammation
- Metformin, a widely prescribed diabetes medication
- Chemotherapy drugs
- Herbal teas containing senna or other natural laxatives
If diarrhea starts within days of beginning a new medication, the medication is a likely suspect.
Chronic Conditions Behind Ongoing Diarrhea
When diarrhea persists for more than four weeks, it’s considered chronic, and a digestive condition is usually the underlying cause. Three of the most common are irritable bowel syndrome (IBS), inflammatory bowel disease (IBD), and celiac disease. They share some symptoms but differ in important ways.
IBS causes stomach pain and changes in bowel habits, including diarrhea, but it does not cause inflammation or physical damage to the digestive tract. It’s a disorder of how the gut functions, not a structural problem. It also doesn’t cause symptoms outside the GI tract.
IBD, which includes Crohn’s disease and ulcerative colitis, is an autoimmune condition that causes real inflammation and physical damage to the digestive system. Unlike IBS, it can affect other parts of the body beyond the gut, and it can prevent your body from absorbing the nutrients it needs. The damage in IBD isn’t limited to one area. Depending on the type, it can affect anywhere from the mouth to the rectum.
Celiac disease is also immune-related but has a specific trigger: gluten, a protein found in wheat, barley, and rye. When someone with celiac disease eats gluten, their immune system attacks the lining of the small intestine, damaging the structures that absorb nutrients. Like IBD, it can lead to nutritional deficiencies, but it differs in that removing gluten from the diet stops the damage.
Acute, Persistent, and Chronic: What Duration Tells You
How long diarrhea lasts is one of the most useful clues to its cause. The American College of Gastroenterology breaks it into three categories: acute diarrhea lasts less than 2 weeks, persistent diarrhea lasts 2 to 4 weeks, and chronic diarrhea lasts more than 4 weeks.
Most acute cases are caused by infections or food-related triggers and resolve on their own. Persistent diarrhea can signal a lingering infection, particularly a parasitic one, or the early phase of a condition that hasn’t been identified yet. Chronic diarrhea almost always points to an underlying condition like IBS, IBD, celiac disease, or a food intolerance that hasn’t been addressed.
Symptoms That Signal Something Serious
Most diarrhea is unpleasant but not dangerous. Certain signs, however, suggest something more serious is going on. In adults, these include diarrhea lasting more than two days, a high fever, six or more loose stools per day, severe abdominal or rectal pain, stools that are black, tarry, or contain blood or pus, and any signs of dehydration like dark urine, dizziness, or extreme thirst.
In children, the timeline is shorter. Diarrhea lasting more than one day warrants a call to the doctor, as does any fever in infants or a child refusing to eat or drink for more than a few hours. Infants under 12 months, premature babies, and children with existing medical conditions are at higher risk of complications and need earlier attention.
Pregnant people, adults over 65, anyone currently taking antibiotics, and those with weakened immune systems are also more vulnerable to dangerous complications from diarrhea and should be in closer contact with their doctor if symptoms develop.