Diarrhea that lasts four weeks or longer is classified as chronic, and it almost always has an identifiable underlying cause. Unlike a stomach bug that clears up in a few days, long-term diarrhea signals that something in your digestive system isn’t working correctly, whether that’s inflammation, a food intolerance, a medication side effect, or a condition that prevents your gut from absorbing nutrients properly.
Irritable Bowel Syndrome
IBS is one of the most common reasons people experience ongoing loose stools. The diarrhea-predominant form (IBS-D) involves frequent urgency, cramping, and watery bowel movements that tend to flare with stress or certain foods. IBS doesn’t cause visible damage to the intestine, which is part of what makes it frustrating to diagnose. It’s typically identified after other conditions have been ruled out.
One important wrinkle: a significant number of people told they have IBS-D actually have a different problem called bile acid malabsorption. A study published in The Lancet’s eClinicalMedicine found that roughly 36 to 38 percent of patients meeting the diagnostic criteria for IBS with diarrhea actually had bile acid malabsorption as the true cause. In this condition, your body produces bile acids to help digest fat but fails to reabsorb them properly in the small intestine. The excess bile acids reach the colon and trigger watery diarrhea. It’s treatable with medication that binds the extra bile acids, but it’s frequently overlooked.
Sugar and Carbohydrate Malabsorption
If your body can’t fully break down or absorb certain sugars, those undigested sugars pull water into the intestine and cause osmotic diarrhea. The most common culprits are lactose (in dairy), fructose (in fruit, honey, and many sweetened foods), and sorbitol (a sugar alcohol found in sugar-free gum, candies, and some fruits like apples and pears). Sucrose intolerance is less common but also possible.
The tricky part is that many people don’t connect their symptoms to a specific food because the reaction can be delayed or dose-dependent. You might tolerate a small amount of milk but have diarrhea after a large bowl of ice cream. A hydrogen breath test can confirm these intolerances. You drink a solution containing the sugar in question, then breathe into a collection device over several hours. Elevated hydrogen in your breath indicates your gut bacteria are fermenting undigested sugar, confirming malabsorption.
Celiac Disease
Celiac disease is an autoimmune condition in which eating gluten (a protein in wheat, barley, and rye) triggers an immune response that damages the lining of the small intestine. That damage reduces your gut’s ability to absorb nutrients, leading to chronic diarrhea, bloating, fatigue, and weight loss. Some people have subtler symptoms like iron deficiency or bone thinning without obvious digestive complaints.
Diagnosis starts with a blood test that looks for specific antibodies your immune system produces in response to gluten. In adults, a biopsy of the small intestine is still required to confirm the diagnosis by showing damage to the tiny finger-like projections that absorb nutrients. In children with very high antibody levels and a positive genetic test, biopsy can sometimes be skipped. The treatment is a strict gluten-free diet, which allows the intestinal lining to heal over weeks to months.
Inflammatory Bowel Disease
Crohn’s disease and ulcerative colitis are the two main forms of inflammatory bowel disease (IBD). Both involve chronic inflammation of the digestive tract, and both commonly cause persistent diarrhea, abdominal pain, and fatigue. Ulcerative colitis affects only the colon and rectum, while Crohn’s can develop anywhere from the mouth to the anus, though it most often targets the end of the small intestine.
An estimated 2.4 to 3.1 million adults in the United States live with IBD. Symptoms often include bloody stool (especially in ulcerative colitis), urgent bowel movements, and periods of flare-ups alternating with remission. Diagnosis involves colonoscopy with biopsies and often imaging studies. Treatment focuses on controlling inflammation to reduce symptoms and prevent long-term damage to the intestine.
Microscopic Colitis
Microscopic colitis is an underrecognized cause of chronic watery diarrhea, particularly in middle-aged and older women. The colon looks completely normal during a colonoscopy, which is why it’s called “microscopic.” The inflammation is only visible when biopsy samples are examined under a microscope. There are two subtypes (collagenous and lymphocytic), but both produce the same main symptom: frequent, non-bloody, watery diarrhea that can persist for months or years if untreated.
Because the colon appears normal on visual inspection, microscopic colitis can only be diagnosed when biopsies are specifically taken during a colonoscopy. Certain medications, particularly proton pump inhibitors and some anti-inflammatory drugs, have been linked to triggering it.
Chronic Infections
Most infections that cause diarrhea resolve within a week or two, but certain parasites can linger for months. Giardia is the most common cause of persistent infectious diarrhea worldwide. It’s typically picked up through contaminated water and causes greasy, foul-smelling stools, bloating, and nausea that can last weeks or longer without treatment. Cryptosporidium and Entamoeba histolytica are other parasites capable of causing prolonged symptoms.
Prolonged diarrhea lasting more than two weeks affects 1 to 3 percent of travelers with diarrhea. In people with weakened immune systems, organisms like Cryptosporidium and microsporidia can cause especially stubborn, debilitating diarrhea that’s difficult to eliminate. A stool test can usually identify the responsible parasite, and targeted treatment clears most infections.
Medications
Several widely used medications cause chronic diarrhea as a side effect, and this possibility is easy to overlook when you’ve been taking a drug for months or years. Metformin, one of the most commonly prescribed diabetes medications, is a frequent offender. It alters the balance of gut bacteria, increases gas production, and affects bile acid processing in the intestine. These effects cause diarrhea in a substantial number of users, sometimes starting months after they begin the medication.
Proton pump inhibitors (PPIs), prescribed for acid reflux, can worsen diarrhea through their own mechanisms and may also amplify the gut effects of other drugs like metformin. Antibiotics are another common trigger because they disrupt the microbial ecosystem of the intestine. Magnesium-containing antacids, certain blood pressure medications, and some antidepressants can also cause ongoing loose stools. If your diarrhea started around the same time as a new medication or dose change, that connection is worth exploring.
Other Causes Worth Considering
Hyperthyroidism (an overactive thyroid) speeds up your metabolism and your digestive tract, causing frequent loose stools along with weight loss, a rapid heartbeat, and anxiety. A simple blood test can rule this in or out. Small intestinal bacterial overgrowth (SIBO), in which bacteria proliferate in the upper part of the gut where they shouldn’t be abundant, causes bloating, gas, and diarrhea by fermenting food before it’s properly absorbed. Chronic pancreatitis, which impairs your body’s ability to produce digestive enzymes, leads to fatty, pale, foul-smelling stools that float.
Certain cancers, particularly colorectal cancer and neuroendocrine tumors, can also present with chronic diarrhea, though this is far less common than the other causes listed above.
Warning Signs That Need Prompt Attention
Most causes of chronic diarrhea are manageable once identified, but certain symptoms alongside diarrhea warrant faster evaluation. These include blood or black color in your stool, unintentional weight loss, fever above 101°F (38°C), severe abdominal or rectal pain, and signs of dehydration like dark urine, dizziness, or extreme thirst. Diarrhea that wakes you from sleep at night is also considered a red flag because functional conditions like IBS rarely disturb sleep, pointing instead toward an inflammatory or structural cause.