Diarrhea happens when your intestines can’t absorb water properly, or when something forces them to push fluid out faster than normal. The causes range from a virus that clears up in a day or two to food intolerances, medications, and chronic digestive conditions that can keep symptoms going for weeks. If loose stools persist three or more times a day for four weeks or longer, that crosses the line from acute to chronic diarrhea, and the underlying cause is likely something beyond a simple stomach bug.
Infections: The Most Common Short-Term Cause
Viruses are the top trigger for sudden-onset diarrhea. Norovirus and rotavirus spread easily through contaminated food, water, or close contact with someone who’s sick. These infections typically cause watery diarrhea along with nausea, vomiting, and sometimes a low fever, and they usually resolve within one to three days.
Bacterial infections tend to hit harder. Salmonella (often from undercooked poultry or eggs), certain strains of E. coli (commonly from contaminated produce or undercooked beef), and Shigella all cause diarrhea by either invading the intestinal lining or releasing toxins that force the gut to secrete fluid. Bacterial diarrhea is more likely to come with cramping, fever, and occasionally bloody stools. Parasites like Giardia, which you can pick up from contaminated water while camping or traveling, cause a more prolonged illness that can last weeks if untreated.
Food Intolerances and Sugar Alcohols
When your body can’t break down a particular substance in food, that substance sits in your intestines and pulls water in through osmosis. The result is loose, watery stools, often with bloating and gas.
Lactose intolerance is the classic example. Your small intestine needs the enzyme lactase to split the sugar in milk into forms your body can absorb. Without enough of that enzyme, lactose passes through undigested, drawing water into the bowel. Fructose, the sugar found in fruit, honey, and high-fructose corn syrup, causes similar problems in people who absorb it poorly.
Sugar alcohols are a particularly sneaky culprit. Sorbitol, the sweetener used in most “sugar-free” gums, candies, and diet products, is poorly absorbed even in healthy people. Studies show that most people experience gas and bloating after just 10 grams of sorbitol, and 20 grams or more reliably causes cramps and diarrhea. A few sticks of sugar-free gum won’t get you there, but a handful of sugar-free candy easily can.
Caffeine and High-Fat Meals
Caffeine stimulates the muscles in your colon, speeding up the rate at which food moves through your digestive tract. It does this partly by blocking certain receptors in the nervous system that would otherwise slow things down, and partly by triggering the release of hormones that increase colonic contractions. For most people, a cup or two of coffee just produces a predictable morning bowel movement. But higher amounts, especially on an empty stomach, can push things into diarrhea territory.
High-fat meals can overwhelm your body’s ability to digest fat efficiently. When undigested fat reaches the large intestine, it draws in water and speeds up transit time. Greasy or fried foods are a frequent trigger for people who already have a sensitive gut, and they’re one of the most commonly reported dietary causes of loose stools.
Medications That Commonly Cause Diarrhea
Nearly any medication can cause diarrhea as a side effect, but certain drug classes do it far more often than others:
- Antibiotics disrupt the balance of bacteria in your gut, which can lead to loose stools during or even after a course of treatment. In some cases, this allows harmful bacteria to overgrow, causing more severe diarrhea.
- Magnesium-containing antacids draw water into the intestines the same way osmotic laxatives do.
- Metformin, one of the most widely prescribed diabetes medications, causes diarrhea in a significant percentage of users, especially when first starting it.
- Heartburn and acid reflux medications (proton pump inhibitors and H2 blockers) can alter gut bacteria and increase the risk of loose stools.
- NSAIDs like ibuprofen and naproxen can irritate the lining of the digestive tract.
- Chemotherapy drugs damage the fast-dividing cells lining the intestines, frequently causing diarrhea as a side effect.
Herbal teas containing senna, a natural laxative, and high-dose vitamin C or magnesium supplements are also common offenders that people don’t always connect to their symptoms.
Chronic Digestive Conditions
When diarrhea keeps coming back or never fully goes away, an underlying condition is usually responsible.
Irritable Bowel Syndrome (IBS)
IBS with diarrhea is one of the most common causes of chronic loose stools. The intestines look structurally normal, but the gut-brain connection is disordered, meaning the muscles of the intestinal wall contract too forcefully or too frequently. Stress, certain foods, and hormonal changes can all trigger flare-ups. The hallmark of IBS is that symptoms tend to improve after a bowel movement and fluctuate over time rather than steadily worsening.
Inflammatory Bowel Disease
Crohn’s disease and ulcerative colitis involve actual inflammation and damage to the digestive tract. Unlike IBS, these conditions cause visible changes that show up on imaging and biopsies. Diarrhea from inflammatory bowel disease often contains blood or mucus, comes with weight loss or fatigue, and tends to worsen progressively without treatment.
Celiac Disease
In celiac disease, eating gluten (a protein in wheat, barley, and rye) triggers an immune response that damages the lining of the small intestine. This damage flattens the tiny finger-like projections that normally absorb nutrients, leading to malabsorption, diarrhea, bloating, and over time, nutritional deficiencies. Diagnosis involves a blood test for specific antibodies followed by a biopsy of the small intestine. The condition is more common than most people realize and is sometimes misdiagnosed as IBS for years.
How Your Body Produces Diarrhea
Not all diarrhea works the same way, and understanding the two main types can help you figure out what’s going on. Osmotic diarrhea happens when something in your gut is pulling extra water in. This is what happens with lactose intolerance, sorbitol, and magnesium-based antacids. The telltale sign is that it stops when you stop eating or drinking the offending substance. If you fast and the diarrhea goes away, it’s almost certainly osmotic.
Secretory diarrhea is different. Here, your intestinal cells are actively pumping fluid out, usually because a bacterial toxin or a hormonal imbalance is telling them to. Cholera is the extreme example, but toxins from Staph and E. coli work the same way on a smaller scale. The key difference: secretory diarrhea continues even if you stop eating entirely.
Signs of Dehydration to Watch For
The biggest immediate risk from diarrhea isn’t the diarrhea itself but the fluid loss. In adults, early signs of dehydration include extreme thirst, dark-colored urine, urinating less than usual, and feeling dizzy or lightheaded. A simple skin test can help: pinch the skin on the back of your hand and release it. If it doesn’t flatten back to normal right away, you’re likely dehydrated.
In infants and young children, the warning signs look different. No wet diapers for three hours or more, no tears when crying, sunken eyes, and unusual drowsiness all point to dehydration that needs attention quickly. Young children and older adults lose fluid faster and tolerate dehydration less well, so the threshold for concern is lower in those groups.