Persistent diarrhea that lasts four weeks or longer is considered chronic, and it affects roughly 5% of adults at any given time. If you’re dealing with loose, watery stools day after day, the cause is almost always identifiable. The challenge is that dozens of conditions share this symptom, so narrowing it down depends on the specific pattern of your diarrhea and what other symptoms come with it.
How Your Body Produces Chronic Diarrhea
Not all diarrhea works the same way inside your gut, and understanding the basic mechanism behind yours can point toward a cause. Chronic diarrhea generally falls into one of four categories based on what’s going wrong in the intestines.
Watery, high-volume diarrhea happens when your intestines stop absorbing water properly. This type often continues at night and even when you haven’t eaten, which distinguishes it from other forms. Bile acid malabsorption, microscopic colitis, and certain hormonal disorders are common culprits.
Diarrhea triggered by poorly absorbed substances occurs when something you’ve eaten pulls extra water into your intestines. Lactose intolerance is a classic example. If you notice your symptoms improve when you fast or skip certain foods, this pattern fits.
Inflammatory diarrhea involves actual damage to the intestinal lining. Blood or mucus in the stool is a hallmark sign. Inflammatory bowel disease, certain infections, and in rare cases colorectal cancer fall into this category.
Fatty diarrhea produces pale, greasy, unusually foul-smelling stools that may float. This happens when your body can’t properly break down or absorb fats. Celiac disease and problems with the pancreas are the most common reasons.
Irritable Bowel Syndrome With Diarrhea
IBS is one of the most frequent explanations for ongoing diarrhea, especially when all the standard tests come back normal. To qualify as IBS, you need to have had abdominal pain at least one day per week for the last three months, with symptoms starting at least six months before that. The pain has to be connected to bowel movements or changes in stool frequency and consistency.
IBS is a real condition, but it’s also a diagnosis of exclusion. That matters because about one-third of people diagnosed with IBS with diarrhea actually have bile acid malabsorption, a separate and treatable problem where excess bile acids flood the colon and trigger watery stools. If you’ve been told you have IBS but haven’t responded well to typical treatments, bile acid malabsorption is worth investigating specifically.
Food Intolerances and Malabsorption
Your gut may struggle to break down specific carbohydrates, and the result is diarrhea, gas, and bloating that show up reliably after eating trigger foods. Lactose intolerance is the most well-known version, but fructose malabsorption and reactions to sugar alcohols (found in sugar-free gum, protein bars, and many “diet” products) are also widespread. Celiac disease, an immune reaction to gluten, damages the lining of the small intestine and leads to both diarrhea and poor nutrient absorption. About 1 in 100 people have celiac disease, and many go years without a diagnosis because their symptoms are attributed to IBS or stress.
A useful clue: if your diarrhea improves noticeably when you stop eating a particular food for two to three weeks, then returns when you reintroduce it, you’ve likely found a trigger. Keeping a food diary for even a week or two can reveal patterns that are otherwise easy to miss.
Inflammatory Bowel Disease
Crohn’s disease and ulcerative colitis are the two main forms of inflammatory bowel disease, and both can cause persistent diarrhea. Ulcerative colitis typically presents with bloody diarrhea, urgent trips to the bathroom, and a frustrating feeling of incomplete emptying even right after a bowel movement. Crohn’s disease can affect any part of the digestive tract and often causes diarrhea alongside abdominal pain, fatigue, and unintended weight loss.
The key difference between IBD and IBS is inflammation. IBD involves measurable, visible damage to the intestinal lining. A stool test for a protein called calprotectin is one of the simplest ways to check: levels below 50 micrograms per gram make IBD very unlikely. Levels above that threshold typically lead to a colonoscopy for a closer look.
Microscopic Colitis
This condition deserves its own mention because it’s surprisingly common and frequently overlooked. Microscopic colitis causes persistent watery diarrhea, but the colon looks completely normal during a standard colonoscopy. The inflammation is only visible under a microscope, which means a biopsy has to be taken during the procedure for the diagnosis to be made. It’s most common in women and in people over 50. If you fit that profile and have unexplained watery diarrhea that doesn’t match any other diagnosis, ask specifically about microscopic colitis, because it can be missed if biopsies aren’t taken routinely.
Medications That Cause Ongoing Diarrhea
Medications are one of the most underappreciated causes of chronic diarrhea, partly because the connection isn’t always obvious. Metformin, widely prescribed for type 2 diabetes, is one of the most common offenders. Diarrhea can start weeks or even months after beginning the medication, making it easy to assume the problem is something else entirely.
Other frequent culprits include antibiotics (which disrupt gut bacteria), proton pump inhibitors used for acid reflux, magnesium-containing antacids, and certain blood pressure medications. If your diarrhea started within a few months of beginning a new medication, that timing is worth flagging. Switching to an extended-release version or an alternative drug often resolves the problem.
Bacterial Imbalance in the Small Intestine
Small intestinal bacterial overgrowth, or SIBO, happens when bacteria that normally live in the large intestine migrate upward and multiply in the small intestine. This can cause diarrhea, bloating, and gas, and in severe cases leads to poor nutrient absorption and weight loss. Recent research suggests the symptoms may come less from the sheer number of bacteria and more from a shift in the types of bacteria present in the small intestine.
SIBO is typically tested with a breath test, though the accuracy of these tests is debated. Transit time through the gut varies so much between individuals that results can be misleading. SIBO is more common in people who’ve had abdominal surgery, who take acid-suppressing medications long term, or who have conditions that slow gut motility.
Patterns That Help Identify the Cause
Paying attention to specific details about your diarrhea gives you and your doctor a significant head start. Consider these questions:
- Does it wake you up at night? Nighttime diarrhea suggests an organic cause like IBD or bile acid malabsorption rather than IBS, which rarely disrupts sleep.
- Does it stop when you fast? Diarrhea that improves when you don’t eat points toward food intolerance or malabsorption. Diarrhea that continues regardless of eating suggests a secretory cause.
- Is there blood or mucus? This signals inflammation and needs prompt evaluation. Black stools can indicate bleeding higher up in the digestive tract.
- Are you losing weight without trying? Unintentional weight loss alongside chronic diarrhea raises concern for malabsorption, IBD, or in rarer cases, cancer.
- Are stools greasy or pale? Fatty stools point toward celiac disease or pancreatic insufficiency.
What Getting Diagnosed Looks Like
The initial workup for chronic diarrhea is usually straightforward. Blood tests check for signs of inflammation, anemia, thyroid dysfunction, and celiac disease. A stool sample can measure calprotectin (to screen for IBD), look for infections, and check for excess fat. These basic tests rule out or confirm a large number of possible causes without any invasive procedures.
If those results don’t point to a clear answer, a colonoscopy with biopsies is often the next step, particularly to check for microscopic colitis or IBD. Breath tests may be used to evaluate for lactose intolerance or SIBO. The process can feel slow, but chronic diarrhea is almost always diagnosable with the right sequence of tests. The biggest reason people go years without answers is that testing stops too early or bile acid malabsorption and microscopic colitis aren’t specifically investigated.