Persistently loose stools that last longer than four weeks are classified as chronic diarrhea, and they affect roughly 5% of adults at any given time. If your stool is consistently soft, mushy, or watery, something is speeding up your digestion, reducing water absorption, or both. The good news is that the cause is usually identifiable and treatable once you know where to look.
What “Loose” Actually Means
Doctors use the Bristol Stool Scale, a visual chart with seven types, to standardize what patients mean when they say their stool is “loose.” Types 5, 6, and 7 all fall into the loose-to-liquid range. Type 5 is soft blobs with clear-cut edges. Type 6 is fluffy, mushy pieces with ragged edges. Type 7 is entirely liquid with no solid pieces at all.
All three types indicate that stool is moving through your intestines too quickly for enough water to be reabsorbed. An occasional Type 5 after a large meal or a cup of coffee is normal. If you’re consistently producing Type 6 or 7, or if even your “good days” are Type 5, that pattern points to an underlying issue worth investigating.
How Your Gut Produces Loose Stool
There are two basic mechanisms behind chronically loose stool, and knowing which one applies to you helps narrow the cause. In osmotic diarrhea, something in your gut is pulling extra water into the intestinal space. Poorly absorbed sugars (like lactose if you’re intolerant, or sugar alcohols from “sugar-free” foods) are common culprits. The telltale sign: your symptoms improve or disappear when you fast or stop eating the trigger food.
In secretory diarrhea, your intestinal lining actively pumps fluid into the gut regardless of what you eat. This type tends to persist even when you haven’t eaten for hours, and it often continues at night. If your loose stools wake you up or don’t respond to dietary changes, a secretory process is more likely.
The Most Common Causes
IBS With Diarrhea
Irritable bowel syndrome is the single most common diagnosis for people with chronically loose stools. IBS with diarrhea (IBS-D) involves recurrent abdominal pain tied to bowel movements, along with a pattern of loose or frequent stools. The pain often improves after a bowel movement. Stress, certain foods, and hormonal shifts can all trigger flares. IBS is a “functional” disorder, meaning the gut looks structurally normal on tests but doesn’t work correctly.
Food Intolerances
Lactose intolerance, fructose malabsorption, and sensitivity to FODMAPs (a group of fermentable carbohydrates found in wheat, onions, garlic, and many fruits) are extremely common and frequently undiagnosed. These sugars draw water into your intestines and get fermented by gut bacteria, producing gas, bloating, and loose stool. Many people live with these for years assuming their digestion is just “how they are.”
Medications
Several widely prescribed medications cause chronic loose stools as a side effect. Metformin, used for diabetes, is one of the most common offenders. Heartburn medications that reduce stomach acid, NSAIDs like ibuprofen and naproxen, and immune-suppressing drugs can all do it. If your loose stools started within a few weeks of beginning a new medication, that connection is worth raising with your prescriber.
Bile Acid Malabsorption
This is one of the most underdiagnosed causes of chronic loose stool. Your liver produces bile acids to help digest fat, and your small intestine is supposed to reabsorb most of them. When that recycling process fails, excess bile acids flood the colon and trigger watery diarrhea. A study published in The Lancet’s eClinicalMedicine found that 38% of patients diagnosed with IBS-D or functional diarrhea actually had bile acid malabsorption as the primary cause. That’s more than one in three people being told they have IBS when a different, treatable condition is driving their symptoms. Bile acid binders can dramatically improve symptoms when this is the cause.
Celiac Disease
Celiac disease is an autoimmune reaction to gluten that damages the lining of the small intestine, reducing its ability to absorb nutrients. It affects about 1% of the population, but many cases go undiagnosed for years. Loose, pale, greasy, or unusually foul-smelling stools are characteristic, often accompanied by bloating, fatigue, and unintended weight loss. A blood test can screen for it.
Pancreatic Insufficiency
Your pancreas produces enzymes that break down fat, protein, and carbohydrates. When it doesn’t produce enough (a condition called exocrine pancreatic insufficiency), undigested fat passes into the colon and causes oily, loose stools that may float or be difficult to flush. This is more common in people with a history of heavy alcohol use, chronic pancreatitis, or cystic fibrosis. A simple stool test that measures an enzyme called elastase can detect it: levels above 200 are normal, 100 to 200 suggest moderate insufficiency, and below 100 confirm it.
Patterns That Help Identify the Cause
Paying attention to when and how your symptoms behave gives you useful information before you ever see a doctor. If your stools firm up when you skip meals or fast, an osmotic process (food intolerance, sugar malabsorption) is likely. If loose stools persist regardless of eating, a secretory cause like bile acid malabsorption or a hormonal issue is more probable.
Notice whether symptoms follow specific foods. Dairy, wheat, high-FODMAP vegetables, artificial sweeteners, and alcohol are the most common dietary triggers. A two-week elimination of one category at a time can reveal connections that years of general observation missed. Keep it simple: remove one thing, observe for two weeks, then reintroduce it and see what happens.
Timing matters too. Loose stools that cluster in the morning and settle by afternoon are a classic IBS pattern. Symptoms that wake you from sleep at night point toward something more inflammatory or secretory, since IBS rarely disturbs sleep.
Signs That Need Prompt Evaluation
Most causes of chronic loose stool are manageable and not dangerous, but certain features signal something more serious. Blood in the stool, whether bright red or dark and tarry, needs evaluation. Unintended weight loss of more than a few pounds, diarrhea that consistently wakes you at night, and bulky, greasy, or extremely foul-smelling stools all warrant a visit sooner rather than later. Onset after age 50 in someone without prior bowel issues also raises the bar for testing, since the risk of inflammatory bowel disease, microscopic colitis, and colorectal cancer increases with age.
What Testing Looks Like
Your doctor will likely start with blood work: a complete blood count, inflammatory markers, celiac antibodies, and thyroid function (an overactive thyroid frequently causes loose stools that people don’t connect to the gland). Stool tests can check for infections, inflammation, fat malabsorption, and pancreatic enzyme levels.
If those come back normal and symptoms persist, a colonoscopy may be recommended, particularly to rule out microscopic colitis, which causes chronic watery diarrhea but looks normal to the naked eye. Biopsies taken during the procedure reveal the diagnosis. Testing for bile acid malabsorption is available in some countries through a specialized scan, though in practice many doctors will simply try a bile acid binder to see if symptoms resolve.
Practical Steps You Can Start Now
While you’re sorting out the root cause, a few changes can reduce symptoms. Soluble fiber (found in oats, psyllium husk, bananas, and white rice) absorbs excess water in the colon and adds bulk to loose stool. Insoluble fiber, found in raw vegetables, whole wheat, and bran, can make things worse for some people, so it’s worth reducing it temporarily.
Cutting back on caffeine and alcohol helps, since both stimulate intestinal motility. Eating smaller, more frequent meals reduces the volume hitting your intestines at once. Probiotic strains in the Saccharomyces boulardii and Lactobacillus families have modest evidence for improving stool consistency in some people, though results vary.
Keeping a simple food and symptom diary for even one or two weeks gives you and your doctor far more to work with than memory alone. Note what you ate, when symptoms appeared, and what the stool looked like on the Bristol Scale. That record can compress months of trial and error into a much faster path to answers.