Persistent diarrhea that keeps coming back usually has an identifiable cause, whether it’s a food your body can’t absorb, a medication side effect, or an underlying digestive condition. When loose stools last longer than four weeks, it’s classified as chronic diarrhea, and it affects a surprisingly large number of people. The good news is that most causes are treatable once you figure out what’s driving it.
How Your Gut Produces Diarrhea
Not all diarrhea works the same way, and understanding the basic mechanism behind yours can point toward the cause. There are three main patterns.
The first is when something in your gut pulls extra water into the intestines. This happens with foods or substances your body can’t fully absorb: lactose if you’re intolerant, fructose from fruit or sweeteners, or sugar alcohols found in “sugar-free” products. The unabsorbed material acts like a sponge, drawing water into your bowel and producing watery, urgent stools. This type typically stops when you stop eating the trigger.
The second pattern involves your intestines failing to reabsorb water normally. Bile acids that aren’t properly recycled, certain medications, alcohol use, an overactive thyroid, and even prior surgeries like gallbladder removal can all interfere with water absorption in the colon. This type tends to persist regardless of what you eat.
The third is inflammatory diarrhea, where the intestinal lining itself is damaged or inflamed. Conditions like Crohn’s disease, ulcerative colitis, and certain infections fall into this category. Stools may contain blood or mucus, and you’re more likely to have fever, weight loss, or fatigue alongside the diarrhea.
Irritable Bowel Syndrome (IBS)
IBS is one of the most common explanations for ongoing diarrhea, especially when tests come back normal. It’s classified as a syndrome, not a disease, meaning there’s no visible damage to the intestines and nothing abnormal shows up on imaging or colonoscopy. The hallmark symptoms are chronic abdominal pain that improves after a bowel movement, diarrhea that alternates with constipation (or diarrhea alone in IBS-D), bloating, gas, and sometimes mucus in the stool.
To meet the diagnostic criteria, you need at least 12 weeks of abdominal discomfort over the past year, along with at least two of these features: relief after a bowel movement, a change in how often you go, or a change in stool consistency. IBS does not cause weight loss, bleeding, fever, or anemia. If you have those symptoms, something else is going on.
One important wrinkle: about one-third of people diagnosed with IBS-D actually have bile acid malabsorption as the underlying cause. Their bodies either overproduce bile acids or fail to reabsorb them, and the excess bile irritates the colon. This is testable and treatable, but it’s frequently overlooked. The American Gastroenterological Association recommends testing for bile acid diarrhea in patients with chronic loose stools.
Inflammatory Bowel Disease (IBD)
Unlike IBS, inflammatory bowel disease causes real, measurable damage to the digestive tract. Crohn’s disease and ulcerative colitis are the two main forms. Both can produce persistent diarrhea, but they also cause symptoms IBS doesn’t: unexplained weight loss, bleeding from the rectum, anemia, and fever. IBD also increases the long-term risk of colon cancer.
The key difference is that IBD shows up on diagnostic imaging and biopsies. A stool test called fecal calprotectin can help sort this out early. A large meta-analysis found that this test detects intestinal inflammation with 93% sensitivity and 96% specificity. A normal result (below 50 micrograms per gram) makes IBD unlikely, while an elevated result signals the need for colonoscopy.
Food Intolerances and Dietary Triggers
Some of the most common causes of chronic diarrhea are sitting on your plate. Lactose intolerance is the classic example: your small intestine doesn’t produce enough of the enzyme that breaks down milk sugar, so it passes undigested into the colon, pulls in water, and ferments. The result is bloating, cramps, and loose stools within a few hours of eating dairy.
Fructose, found naturally in fruit and added to many processed foods as high-fructose corn syrup, causes the same osmotic effect in people who absorb it poorly. Celiac disease is another major culprit. It’s an autoimmune reaction to gluten that damages the small intestine’s lining and impairs nutrient absorption. Screening involves a simple blood test for specific antibodies.
Sugar alcohols deserve special attention because they’re increasingly common in packaged foods. Sorbitol, xylitol, mannitol, maltitol, erythritol, and isomalt are all used as low-calorie sweeteners in sugar-free gum, candy, protein bars, and diabetic foods. Your body can’t fully digest them, so they ferment in the gut and draw in water. The FDA requires products containing sorbitol or mannitol to carry a “may cause a laxative effect” warning. People with IBS or Crohn’s disease are especially sensitive to them. Check ingredient labels if your diarrhea seems random: sugar alcohols hide in surprising places, from mouthwash to protein powders.
Medications That Cause Diarrhea
Nearly any medication can cause diarrhea, but certain drug classes do it far more often. If your symptoms started or worsened after beginning a new prescription, the medication itself may be the problem.
- Metformin, the most widely prescribed diabetes drug, causes diarrhea in a significant percentage of users, especially at higher doses or when first starting.
- Antibiotics disrupt the balance of gut bacteria, often producing diarrhea that can persist for weeks after the course ends.
- Acid-reducing drugs like omeprazole, pantoprazole, and famotidine are linked to chronic diarrhea, particularly with long-term use.
- NSAIDs like ibuprofen and naproxen can irritate the gut lining.
- Magnesium-containing antacids work as osmotic laxatives in the colon.
If you suspect a medication, don’t stop it on your own, but do bring it up with whoever prescribed it. Switching formulations, adjusting timing, or trying an alternative often resolves the issue.
Less Obvious Causes Worth Knowing
Bile acid malabsorption is one of the most underdiagnosed causes of chronic diarrhea. Studies show it affects up to 50% of people with unexplained functional diarrhea and about a third of those labeled with IBS-D. After you eat, your liver produces bile acids to help digest fat. Normally, your small intestine reabsorbs most of them. When it doesn’t, they spill into the colon and trigger watery diarrhea, often urgently after meals. Two tests can confirm it: a 48-hour stool collection that measures bile acid levels, or a fasting blood test that measures a marker of bile acid production.
Microscopic colitis is another sneaky one. It causes watery, non-bloody diarrhea, often in middle-aged and older adults. The colon looks completely normal on colonoscopy, but biopsies reveal inflammation visible only under a microscope. It’s easily missed if biopsies aren’t taken.
An overactive thyroid speeds up gut motility, pushing food through before water can be properly absorbed. If your diarrhea comes with unexplained weight loss, a racing heart, or heat intolerance, thyroid function is worth checking.
Symptoms That Signal Something Serious
Most chronic diarrhea turns out to be caused by something manageable, but certain symptoms raise the stakes. Blood in your stool, unintentional weight loss, persistent fever, waking from sleep to have diarrhea (nocturnal diarrhea), and signs of anemia like unusual fatigue or paleness all point toward conditions that need prompt evaluation. Nocturnal diarrhea is particularly telling because IBS almost never wakes you from sleep, so nighttime symptoms suggest an organic disease process like IBD or an infection.
Getting to an Answer
If you’ve had loose stools for more than four weeks, a systematic approach works better than guessing. Start by looking at your diet and medications for obvious triggers: dairy, high-fructose foods, sugar alcohols, new prescriptions. An elimination diet, where you remove one suspect category at a time for two to three weeks, can be revealing.
When dietary changes don’t solve it, basic lab work helps narrow the field. Celiac antibody testing is recommended as a standard step. A fecal calprotectin test can separate inflammatory conditions from functional ones without requiring an invasive procedure. If those come back normal, testing for bile acid malabsorption is a logical next step, given how frequently it’s the hidden culprit.
Keeping a symptom diary that tracks what you eat, when symptoms hit, and what your stools look like gives you and your provider far more to work with than a vague description of “always having diarrhea.” Patterns often emerge within a week or two that point directly to the cause.