Chronic diarrhea, defined as loose or watery stools three or more times a day lasting four weeks or longer, has dozens of possible causes ranging from dietary triggers and medications to inflammatory diseases and infections. Unlike a stomach bug that clears up in a few days, chronic diarrhea signals an ongoing problem that needs identification. The cause falls into one of a few broad categories: something you’re eating or taking, inflammation in the gut, a problem with how your body absorbs nutrients, or a functional disorder where the gut looks normal but doesn’t work properly.
Irritable Bowel Syndrome
IBS with diarrhea is one of the most common explanations, especially in people under 50 with no other alarming symptoms. It’s classified as a functional disorder, meaning the gut looks structurally normal on imaging and during a colonoscopy, but the nerves and muscles controlling digestion don’t coordinate well. People with IBS tend to have heightened sensitivity in the gut wall, so normal amounts of gas or movement register as pain or urgency.
Because there’s no single test that confirms IBS, it’s typically diagnosed after ruling out other causes. A stool marker called fecal calprotectin can help sort this out. Levels below 40 micrograms per gram generally indicate no significant inflammation, pointing toward IBS rather than an inflammatory disease. Levels above 100 suggest active inflammation and warrant further investigation. The gray zone between 40 and 100 is harder to interpret and often requires additional testing.
Inflammatory Bowel Disease
Crohn’s disease and ulcerative colitis cause chronic diarrhea through ongoing inflammation that damages the intestinal lining. Unlike IBS, the inflammation in IBD is visible and measurable. Even when someone with IBD feels fine between flares, their gut tissue often still shows elevated inflammatory markers compared to someone with IBS. This persistent, smoldering inflammation disrupts the intestine’s ability to absorb water and nutrients properly, which keeps stools loose.
Blood in the stool, unexplained weight loss, fatigue, and joint pain are common alongside the diarrhea. A family history of IBD, celiac disease, or colorectal cancer raises the likelihood that chronic diarrhea has an inflammatory cause rather than a functional one.
Microscopic Colitis
This is a sneaky one. Microscopic colitis causes frequent, watery, non-bloody diarrhea, often with urgency, bloating, and fatigue. It shows up most often in adults over 60 and is frequently misdiagnosed as IBS because the two look almost identical from the outside. Even a standard colonoscopy appears normal.
The only way to diagnose it is through tissue biopsies taken during a colonoscopy and examined under a microscope, where a distinct pattern of inflammation becomes visible. Current guidelines recommend at least six biopsies from different parts of the colon to catch it reliably. Fecal incontinence, weight loss, and joint pain during flares are additional clues, particularly in older adults who develop new-onset diarrhea without an obvious explanation.
Food Intolerances and Dietary Triggers
What you eat can absolutely sustain chronic diarrhea, and the culprits aren’t always obvious. Lactose intolerance is well known, but sugar alcohols found in sugar-free gums, candies, protein bars, and “diet” products are a major and underrecognized trigger. Sorbitol, one of the most common sugar alcohols, can cause diarrhea at surprisingly low doses: roughly 0.17 grams per kilogram of body weight in men and 0.24 grams per kilogram in women. For a 70-kilogram (154-pound) man, that’s only about 12 grams of sorbitol, an amount easily reached by chewing several pieces of sugar-free gum in a day.
These sweeteners work by pulling water into the intestine, a process called osmotic diarrhea. Your body can’t fully absorb them, so they sit in the gut, draw in fluid, and speed everything through. Erythritol has a higher tolerance threshold (roughly three to four times that of sorbitol), but people with IBS are more sensitive to all sugar alcohols and may react at lower doses. Fructose in large amounts, from fruit juices, honey, or high-fructose corn syrup, works the same way.
Celiac disease deserves its own mention here. It’s an autoimmune reaction to gluten that damages the small intestine and impairs nutrient absorption. It affects roughly 1 in 100 people, many of whom go undiagnosed for years. Chronic diarrhea, bloating, and unintentional weight loss are hallmark symptoms, and a blood test for specific antibodies can screen for it before a biopsy confirms it.
Medications That Cause Chronic Diarrhea
Medications are one of the most overlooked causes. If your diarrhea started around the time you began a new drug or changed a dose, that connection is worth investigating. Common offenders include:
- Metformin, widely prescribed for type 2 diabetes, causes persistent loose stools in a significant number of users.
- Proton pump inhibitors (heartburn medications like omeprazole, lansoprazole, and pantoprazole) can alter gut bacteria and cause diarrhea with long-term use.
- NSAIDs like ibuprofen and naproxen irritate the gut lining and can trigger diarrhea, especially with regular use.
- Antibiotics disrupt the balance of gut bacteria, sometimes causing diarrhea that persists well after the course is finished.
- Magnesium-containing antacids draw water into the intestine the same way sugar alcohols do.
- Herbal teas and supplements containing senna or other natural laxatives, sometimes not clearly labeled as such.
Chemotherapy drugs and immune-suppressing medications also frequently cause diarrhea, though people taking those are usually aware of the connection.
Bile Acid Diarrhea
Your liver produces bile acids to help digest fat. Normally, the gallbladder stores and concentrates bile, releasing it in controlled amounts when you eat. After gallbladder removal, that storage system is gone. Bile drips continuously into the small intestine in a diluted, uncontrolled flow. When a meal triggers normal digestive reflexes, large amounts of bile get pushed into the colon, where excess bile acids stimulate the lining to secrete water and electrolytes, producing diarrhea.
One multicenter study found that diarrhea after gallbladder removal affected over 57% of patients, though only a small proportion were formally investigated for it, and diagnosis was often delayed. Bile acid diarrhea can also occur without gallbladder surgery, due to problems with reabsorption in the lower small intestine, as seen in Crohn’s disease or after bowel resection surgery.
Infections That Don’t Resolve
Most infectious diarrhea clears within days, but certain parasites can establish chronic infections. Giardia, contracted through contaminated water or food, is a classic cause of diarrhea lasting weeks to months. Cryptosporidium, the leading cause of waterborne diarrheal disease in the United States, typically resolves on its own in healthy people but can cause prolonged illness in anyone with a weakened immune system.
Small intestinal bacterial overgrowth (SIBO) is another possibility. When bacteria that normally live in the large intestine colonize the small intestine, they ferment food prematurely, producing gas, bloating, and diarrhea. This is more common in people with diabetes, prior abdominal surgery, or conditions that slow intestinal movement.
Post-infectious IBS is also real. After a bad bout of food poisoning or traveler’s diarrhea, some people develop IBS-like symptoms that persist for months or even years. The infection resolves, but the gut’s sensitivity and motility patterns remain disrupted.
Exocrine Pancreatic Insufficiency
The pancreas produces enzymes that break down fat, protein, and carbohydrates. When it can’t produce enough of these enzymes, food passes through partially undigested. The result is chronic diarrhea with stools that are pale, greasy, foul-smelling, and tend to float. This is most commonly seen in people with a history of chronic pancreatitis, cystic fibrosis, or pancreatic surgery, but it can also develop in older adults without an obvious cause.
Red Flag Symptoms Worth Noting
Certain features of chronic diarrhea point toward a more serious underlying cause that needs prompt evaluation. These include diarrhea that wakes you from sleep at night, unintentional weight loss, blood in the stool or unexplained iron deficiency, severe or progressively worsening symptoms, and new onset of symptoms after age 50. A family history of IBD, celiac disease, or colorectal cancer also elevates the concern. None of these features alone confirm a diagnosis, but they help distinguish cases that need colonoscopy, imaging, or blood work from those more likely to be functional or dietary.