Constant diarrhea, meaning loose or watery stools lasting longer than four weeks, is classified as chronic diarrhea. It’s not a diagnosis on its own but a symptom pointing to an underlying cause, and the list of possible causes is long. The most common include irritable bowel syndrome, food intolerances, medications, inflammatory bowel disease, and infections that haven’t fully cleared. Narrowing it down depends on what your stools look like, what other symptoms you have, and how your body responds to eating and fasting.
Three Types of Chronic Diarrhea
Not all diarrhea works the same way inside your body, and the type you have points toward different causes. Doctors generally group chronic diarrhea into three broad categories based on what’s happening in the gut.
Watery diarrhea is the most common type. It splits further into osmotic and secretory. Osmotic diarrhea happens when something in your gut pulls extra water into the intestines, like an undigested sugar (lactose, fructose, sorbitol). A key feature: it gets better when you stop eating. Secretory diarrhea is the opposite. Your intestines actively push fluid out even when you’re fasting. This type is linked to infections, overactive thyroid, microscopic colitis, bile salt problems, and certain medications.
Fatty diarrhea produces bulky, oily stools that float and are difficult to flush. It signals that your body isn’t absorbing or digesting fats properly. Celiac disease, pancreatic insufficiency, bile acid malabsorption, and bacterial overgrowth in the small intestine are the usual suspects. Weight loss often accompanies it because nutrients are passing through without being absorbed.
Inflammatory diarrhea involves frequent, small-volume stools that may contain blood or mucus. It’s often accompanied by fever, urgent cramping, and a painful sensation of needing to go even when the bowel is empty. Crohn’s disease, ulcerative colitis, radiation damage to the gut, and certain invasive infections cause this pattern.
Irritable Bowel Syndrome
IBS is one of the most common reasons for ongoing diarrhea, affecting roughly 11% of the global population. The diarrhea-predominant form (IBS-D) causes recurring loose stools along with abdominal pain, cramping, and bloating. Symptoms tend to flare with stress, certain meals, or hormonal changes, and they often improve after a bowel movement.
IBS is a functional disorder, meaning the gut looks structurally normal on tests but doesn’t work the way it should. There’s no single blood test or scan that confirms it. Instead, it’s diagnosed when your symptoms fit a specific pattern and other conditions have been ruled out. About one in three people with IBS also have fructose malabsorption, where the small intestine can’t fully absorb the natural sugar found in fruits, honey, and many processed foods. Unabsorbed fructose draws water into the intestines, triggering diarrhea and gas.
Food Intolerances and Sugar Malabsorption
If your diarrhea worsens after meals and improves when you skip eating, a food intolerance is a strong possibility. Lactose intolerance is the most recognized, but fructose and sorbitol malabsorption are surprisingly common and often overlooked. Sorbitol is an artificial sweetener found in sugar-free gum, mints, and some diet products. Even small amounts can cause watery diarrhea in sensitive people.
The mechanism is straightforward. When your small intestine can’t absorb a sugar, it travels to the large intestine intact. There, it changes the osmotic pressure, pulling water into the bowel and feeding bacteria that produce gas. The result is bloating, cramping, and loose stools, typically within a few hours of eating the trigger food. An elimination diet, where you remove suspected triggers for two to three weeks and then reintroduce them one at a time, is often the most practical way to identify the culprit.
Medications That Cause Diarrhea
Medications are an underappreciated cause of chronic diarrhea, and the connection isn’t always obvious. Metformin, the most widely prescribed drug for type 2 diabetes, is well known for causing gastrointestinal side effects when treatment starts. But it can also trigger late-onset diarrhea in people who previously tolerated it without problems, sometimes years into treatment. The likely mechanism involves increased intestinal motility and changes in bile salt absorption. Older adults on multiple medications face a higher risk because drug interactions can raise metformin levels in the blood.
Antacids, proton pump inhibitors, antibiotics, magnesium supplements, and weight-loss drugs like orlistat are other frequent offenders. If your diarrhea started or worsened around the time you began a new medication, or if a dose changed, that’s worth flagging to your provider. Stopping or switching the drug often resolves the problem within days.
Inflammatory Bowel Disease
Crohn’s disease and ulcerative colitis are autoimmune conditions where the immune system attacks the lining of the digestive tract. Both cause chronic diarrhea, but they look different. Ulcerative colitis typically causes bloody diarrhea with urgency and is limited to the colon. Crohn’s can affect any part of the digestive tract, often causing diarrhea along with fatigue, weight loss, and abdominal pain that may come and go over months.
A stool test measuring a protein called calprotectin helps distinguish IBD from functional conditions like IBS. Levels below 150 micrograms per gram generally rule out active inflammation. Elevated levels prompt further investigation with a colonoscopy. IBD is a lifelong condition, but modern treatments can bring symptoms into remission for long stretches.
Bile Acid Diarrhea
Your liver produces bile acids to help digest fat. Normally, most bile acids are reabsorbed in the small intestine and recycled. When that recycling system breaks down, excess bile acids flood the colon and act as a laxative, pulling water in and speeding up transit.
This is especially common after gallbladder removal. Without the gallbladder to store and regulate bile release, bile drips continuously into the intestine, and the colon gets more bile acids than it can handle. The result is watery, urgent diarrhea, often shortly after meals. Bile acid malabsorption also occurs without prior surgery in conditions like Crohn’s disease or as a standalone problem. Medications called bile acid binders, such as cholestyramine, can soak up the excess and relieve symptoms significantly.
Infections That Linger
Most stomach bugs clear within a week, but certain parasites are built to persist. Giardia is the most common cause of prolonged infectious diarrhea worldwide. It’s transmitted through contaminated water or food and causes watery, foul-smelling stools, bloating, and nausea that can last weeks or months without treatment. Cryptosporidium is the second most common parasitic cause and tends to be more severe in people with weakened immune systems.
Other parasites capable of causing chronic symptoms include Entamoeba (the cause of amoebic dysentery), Cyclospora, and Strongyloides. Travelers to tropical or developing regions are at highest risk, but waterborne outbreaks happen everywhere. Clostridioides difficile, a bacterial infection often triggered by antibiotic use, is another important cause, particularly in older adults or people who’ve been hospitalized recently. Stool testing for ova and parasites, ideally collected on multiple separate days, is the standard way to catch these infections.
Warning Signs That Need Attention
Most causes of chronic diarrhea are manageable and not dangerous, but certain patterns signal something more serious. Blood or black tarry stools suggest bleeding somewhere in the digestive tract. Unintentional weight loss, especially combined with diarrhea, points toward malabsorption or inflammatory disease. Diarrhea that wakes you from sleep at night is a red flag because functional conditions like IBS almost never do this.
Severe dehydration is the most immediate risk from any prolonged diarrhea. Signs include dark urine, reduced urination, dizziness when standing, and extreme thirst. Over time, chronic diarrhea can also lead to nutritional deficiencies, causing anemia, weakened immunity, and fatigue. Fever alongside diarrhea suggests an infectious or inflammatory process that needs investigation rather than watchful waiting.
How the Cause Gets Identified
The first step is usually a detailed history: when the diarrhea started, whether it improves with fasting, what your stools look like, and what medications you take. From there, initial testing often includes stool cultures to check for bacterial infections, ova and parasite exams, and a stool calprotectin test to screen for inflammation. Blood work can reveal celiac disease, thyroid dysfunction, or signs of malabsorption like low iron or vitamin levels.
If those initial tests come back normal and symptoms persist, a colonoscopy or upper endoscopy may follow to look for microscopic colitis, IBD, or celiac damage. Breath tests can identify lactose or fructose malabsorption. The process can feel slow, but it works by elimination, ruling out the most serious possibilities first and then zeroing in on the most likely explanation for your specific pattern of symptoms.