Diarrhea in adults is most often caused by viral infections, food intolerances, or medications. When it lasts fewer than four weeks, it’s considered acute and typically resolves on its own. Diarrhea lasting longer than four weeks is classified as chronic and usually points to an underlying condition that needs investigation.
Infections: The Most Common Short-Term Cause
Viruses are the leading cause of acute diarrhea in adults. Norovirus alone is responsible for the majority of non-bacterial gastroenteritis outbreaks, spreading rapidly through contaminated food, water, and surfaces. Symptoms usually hit fast, within 12 to 48 hours of exposure, and clear up in one to three days.
Bacterial infections from Salmonella, Campylobacter, and E. coli tend to come from undercooked meat, contaminated produce, or unsafe water. These episodes are often more intense than viral ones, sometimes producing bloody stool or high fevers. Parasites like Giardia are less common but can drag on much longer. Giardia has an incubation period of 1 to 14 days (7 days on average) and typically causes symptoms for one to three weeks. Travel to areas with untreated water supplies is a major risk factor for parasitic diarrhea.
One important wrinkle: roughly 1 in 5 adults who recover from a bacterial gut infection like Campylobacter go on to develop lasting bowel symptoms that meet the criteria for irritable bowel syndrome. So an infection that seemed to resolve can sometimes leave behind chronic digestive problems.
Medications That Trigger Diarrhea
Drug-induced diarrhea is extremely common and often overlooked. Antibiotics are the most well-known culprits. They disrupt the balance of bacteria in your gut, which can lead to loose stools during or even after a course of treatment. But the list of medications that cause diarrhea is surprisingly long:
- Magnesium-containing antacids draw water into the intestines, loosening stool.
- Heartburn and ulcer medications (proton pump inhibitors and H2 blockers) can alter gut flora and acid levels enough to cause diarrhea.
- NSAIDs like ibuprofen and naproxen irritate the gut lining.
- Metformin, a widely prescribed diabetes medication, causes diarrhea in a significant percentage of users, especially when first starting treatment.
- Chemotherapy drugs damage fast-dividing cells in the intestinal lining.
- Herbal teas and supplements containing senna or other natural laxatives can trigger diarrhea without the user realizing they’ve essentially taken a laxative.
If your diarrhea started around the same time as a new medication or supplement, that connection is worth exploring with your prescriber.
Food Intolerances and Malabsorption
When your small intestine can’t fully absorb certain sugars, they sit in the gut, pull water into the intestinal space through osmotic pressure, and get fermented by bacteria. The result is diarrhea, bloating, gas, and cramping, often within a few hours of eating the trigger food.
Lactose intolerance is the most recognized version of this, but fructose malabsorption is surprisingly widespread. The transport system that moves fructose across the intestinal wall has a limited capacity, and when it gets overloaded, the excess fructose stays in the gut and causes symptoms. Fruits with high levels of excess free fructose, particularly apples, pears, mangoes, and Asian pears, are the most likely to trigger problems. High fructose corn syrup, found in soft drinks and many processed foods, is another major source.
Sugar alcohols like sorbitol and mannitol, common in sugar-free gums and candies, work through the same osmotic mechanism. Fructans, a type of carbohydrate found in onions, asparagus, wheat, and Jerusalem artichokes, can also be poorly absorbed and fermented in the gut. People who notice diarrhea after meals but can’t pin it on a single food may be reacting to one of these less obvious triggers.
IBS vs. Inflammatory Bowel Disease
Two chronic conditions frequently cause recurring diarrhea in adults, and they’re often confused despite being fundamentally different.
Irritable bowel syndrome (IBS) is a syndrome, meaning it’s defined by a pattern of symptoms rather than visible damage to the intestines. To meet the diagnostic criteria, you need to have had abdominal discomfort or pain for at least 12 weeks 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. Colonoscopies and imaging look completely normal in people with IBS. It does not cause inflammation, bleeding, or weight loss, and it does not increase your risk for colon cancer.
Inflammatory bowel disease (IBD), which includes Crohn’s disease and ulcerative colitis, is a disease that causes destructive inflammation visible on imaging and during colonoscopy. IBD can produce symptoms that IBS never does: bloody stool, fever, anemia, and significant weight loss. It carries an increased risk for colon cancer and sometimes requires surgery. If you’re experiencing diarrhea alongside any of those warning signs, that distinction matters.
Bile Acid Malabsorption
Your liver produces bile acids to help digest fat. Normally, the end of your small intestine reabsorbs about 95% of those bile acids and recycles them. When that reabsorption process fails, excess bile acids flood into the colon, triggering watery diarrhea, urgency, painful cramps, and sometimes fatty or greasy stools.
Bile acid malabsorption can develop on its own or alongside other conditions, including Crohn’s disease, microscopic colitis, and pancreatic insufficiency. It can also persist after a bacterial gut infection has cleared. The hallmark symptoms are frequent watery stools with a strong sense of urgency, meaning difficulty holding bowel movements in. Over time, the chronic diarrhea can lead to dehydration, fatigue, dizziness, and weight loss.
How Diarrhea Actually Works in the Gut
Not all diarrhea involves the same mechanism, and understanding the difference helps explain why certain triggers produce certain symptoms. Osmotic diarrhea happens when something in your intestine pulls water in. Unabsorbed sugars like lactose or fructose are classic examples, as are magnesium-based antacids. This type of diarrhea typically stops when you stop eating (or taking) the trigger.
Secretory diarrhea is different. The cells lining your intestinal crypts actively pump chloride ions into the gut lumen, and water follows. Certain bacterial toxins, hormonal imbalances, and some medications can flip this switch. Secretory diarrhea tends to continue even when you’re fasting, which is one way doctors distinguish it from osmotic causes.
Warning Signs That Need Attention
Most acute diarrhea runs its course in a day or two. But certain symptoms signal something more serious. Bloody or black stools suggest bleeding somewhere in the digestive tract. A fever above 102°F (39°C) points toward a significant infection. Signs of dehydration, including excessive thirst, dark urine, dry mouth, dizziness, or severe weakness, mean your body is losing fluid faster than you can replace it. Severe diarrhea, defined as more than 10 bowel movements a day or fluid losses clearly exceeding what you can drink, can become dangerous quickly.
Diarrhea that persists beyond two days without any improvement, or that comes with severe abdominal or rectal pain, also warrants medical evaluation. And any diarrhea lasting longer than four weeks has crossed into chronic territory, where identifying the underlying cause becomes essential rather than optional.