Constant diarrhea, defined as loose stools lasting more than four weeks, can stem from dozens of causes ranging from food intolerances and medications to infections and inflammatory diseases. Unlike a stomach bug that clears up in a few days, persistent diarrhea signals that something ongoing is disrupting how your gut absorbs water or moves food through your system. Pinpointing the cause matters because many of these conditions are highly treatable once identified.
Food Intolerances and Malabsorption
One of the most common causes of chronic loose stools is an inability to properly absorb certain sugars. Lactose intolerance is the classic example: your small intestine doesn’t produce enough of the enzyme needed to break down lactose, the sugar in milk. The undigested lactose stays in the intestine, where it pulls water in by osmosis and holds it there. That excess water is what makes stools loose. To make things worse, the unabsorbed sugar then passes into the colon, where bacteria ferment it, producing gas and bloating on top of the diarrhea.
Lactose isn’t the only culprit. Fructose, found in fruit juice, honey, and many processed foods, causes the same problem in people who absorb it poorly. Sugar alcohols like sorbitol and mannitol, common in sugar-free gum and diet products, are notorious for triggering osmotic diarrhea because they’re poorly absorbed by almost everyone in large enough quantities. If your diarrhea seems to flare after specific foods or drinks, a pattern of malabsorption is worth investigating.
Celiac disease is another malabsorption condition that causes chronic diarrhea. In people with celiac disease, gluten (a protein in wheat, barley, and rye) triggers an immune response that damages the lining of the small intestine, impairing its ability to absorb nutrients across the board. The result is persistent diarrhea along with fatigue, weight loss, and nutritional deficiencies.
Medications That Keep the Gut Running
Nearly any medication can cause diarrhea as a side effect, but certain classes do so frequently enough that they deserve a hard look if your symptoms started around the time you began a new prescription. Antibiotics are a well-known trigger because they disrupt the balance of bacteria in your gut, sometimes allowing problem organisms like C. difficile to take over. That disruption can persist for weeks or months after finishing the antibiotic course.
Metformin, one of the most widely prescribed diabetes medications, causes chronic diarrhea in a significant number of people who take it. Proton pump inhibitors (medications for heartburn and acid reflux like omeprazole and lansoprazole) can also be responsible, as can over-the-counter pain relievers like ibuprofen and naproxen. Magnesium-containing antacids work by drawing water into the intestine, essentially acting as mild laxatives. Even herbal teas containing senna, often marketed as “detox” or “cleansing” teas, contain natural laxative compounds that cause diarrhea with regular use.
If you’ve been taking any of these for a while and can’t identify another cause, the medication itself is a strong suspect.
Irritable Bowel Syndrome and Bile Acid Issues
IBS with diarrhea (IBS-D) is one of the most frequently diagnosed causes of chronic loose stools. It’s considered a “functional” disorder, meaning the gut looks structurally normal but doesn’t function the way it should. Symptoms typically include cramping, urgency, and diarrhea that worsens with stress or certain foods.
Here’s the complication: a substantial portion of people diagnosed with IBS-D actually have an underlying problem with bile acids that’s going unrecognized. Your liver produces bile acids to help digest fat, and normally your intestine reabsorbs most of them. When that reabsorption fails, excess bile acids flood the colon and stimulate it to secrete water, producing watery diarrhea. Research from the Mayo Clinic has shown that bile acid malabsorption occurs in roughly one-third of patients diagnosed with IBS-D and up to 50 percent of those labeled with “functional diarrhea.” That’s a significant number of people whose diarrhea has a specific, treatable mechanism hiding behind a more general diagnosis.
Infections That Linger
Most infectious diarrhea is acute, caused by viruses or bacteria that clear within days. But certain parasites are built to persist. Giardia is the prime example. It’s common enough that the American Gastroenterological Association recommends testing for it in every patient with chronic diarrhea. Giardia infections typically begin one to two weeks after exposure and last two to six weeks, but in some people symptoms drag on for years if untreated. You can pick it up from contaminated water, including streams and lakes that look pristine.
Cryptosporidium is another parasite capable of causing prolonged diarrhea, particularly in people with weakened immune systems. Small intestinal bacterial overgrowth (SIBO), where bacteria proliferate in a part of the gut where they normally exist in low numbers, can also produce ongoing diarrhea along with bloating and discomfort.
Inflammatory Bowel Disease
Crohn’s disease and ulcerative colitis are chronic inflammatory conditions that directly damage the intestinal lining. Unlike IBS, these diseases cause visible inflammation and ulceration. Diarrhea from IBD often contains blood or mucus, and it may wake you up at night, a feature that distinguishes it from most functional causes. Weight loss, fatigue, and abdominal pain are common companions.
Microscopic colitis is a less well-known form of inflammatory bowel disease that causes chronic watery diarrhea without visible damage on standard colonoscopy. It’s only detected through tissue biopsies examined under a microscope, which is how it gets its name. It’s more common in older adults and in people taking certain medications like NSAIDs and proton pump inhibitors. About 35 percent of people with microscopic colitis also have bile acid malabsorption contributing to their symptoms.
Hormonal and Endocrine Causes
An overactive thyroid gland speeds up many body processes, including how fast food moves through your gut. Hyperthyroidism can cause frequent loose stools along with weight loss, anxiety, a rapid heartbeat, and heat intolerance. If your diarrhea came on alongside those other symptoms, thyroid function is worth checking.
Rarer but important to know about are neuroendocrine tumors, sometimes called carcinoid tumors. These slow-growing tumors, most commonly found in the digestive tract or lungs, can secrete hormones like serotonin and histamine that trigger chronic diarrhea and facial flushing. This combination of flushing and diarrhea is called carcinoid syndrome. These tumors are uncommon, but they illustrate why persistent, unexplained diarrhea deserves a thorough evaluation.
Diabetes itself can cause chronic diarrhea through nerve damage that affects how the gut contracts and moves food, independent of any medication effect.
How Doctors Figure Out the Cause
Diagnosing chronic diarrhea usually starts with your history: when it began, what makes it better or worse, whether it happens at night, and what medications you take. From there, a few targeted tests narrow things down efficiently.
Stool tests for markers of intestinal inflammation (fecal calprotectin or fecal lactoferrin) help screen for inflammatory bowel disease without needing a colonoscopy right away. A Giardia-specific stool test is recommended as a standard first step. Blood work can check for celiac disease, thyroid problems, and signs of malabsorption like anemia or low vitamin levels.
If initial tests don’t point to a clear answer, a colonoscopy with biopsies may be needed, particularly to catch microscopic colitis or confirm IBD. Testing for bile acid malabsorption is available but underutilized in many settings, which partly explains why so many people with this condition go undiagnosed for years.
Warning Signs That Need Prompt Attention
Certain features of chronic diarrhea suggest a more serious underlying cause. Blood in the stool, whether bright red or black and tarry, always warrants investigation. Unintentional weight loss, persistent fever, and diarrhea that wakes you from sleep at night all point away from a benign functional cause. Severe abdominal pain, signs of dehydration (dizziness, dark urine, dry mouth), and six or more loose stools per day also raise the urgency for evaluation.