Diarrhea happens when something disrupts the way your intestines absorb water, pushing loose or watery stool through faster than normal. The cause ranges from a stomach bug that resolves in a day or two to a food intolerance you haven’t identified yet to a medication side effect you didn’t expect. Most episodes are acute, meaning they clear up on their own within two weeks, but understanding the pattern and timing helps you figure out what’s going on.
A Stomach Bug Is the Most Common Cause
Viral infections are responsible for the majority of sudden-onset diarrhea. Norovirus is the leading culprit in adults. It spreads through direct contact, contaminated food, or the fecal-oral route, and symptoms typically hit 24 to 48 hours after exposure. You’ll often have nausea and vomiting alongside the diarrhea. Outbreaks peak from November through April in the United States but can happen year-round. Rotavirus is the more common cause in young children, especially between 3 and 15 months of age, with an incubation period of one to three days. Widespread vaccination has reduced its impact significantly.
Bacterial infections tend to be more intense. Salmonella and Campylobacter are the most frequent bacterial causes in the U.S., usually picked up from undercooked poultry, unpasteurized milk, or (in the case of Salmonella) undercooked eggs and contact with reptiles or birds. These infections are more likely to produce fever, exhaustion, and bloody stool compared to viral infections.
Parasites like Giardia are less common but worth knowing about, especially if your diarrhea drags on for weeks. Giardia is picked up from contaminated water or person-to-person spread (day care centers are a frequent setting). It clings to the intestinal lining and can become chronic, eventually interfering with your body’s ability to absorb nutrients.
Something You Ate (or Can’t Digest)
Food intolerances are a sneaky and extremely common source of recurring diarrhea. Lactose intolerance is the most widespread: your body doesn’t produce enough of the enzyme needed to break down the sugar in milk, cheese, and yogurt. The undigested lactose pulls water into the intestine, producing bloating, gas, cramps, and loose stool, usually within a few hours of eating dairy.
Gluten intolerance is another well-known trigger. In celiac disease, eating gluten (found in wheat, rye, and barley) causes an immune reaction that damages the lining of the small intestine. Over time, this damage prevents your body from absorbing the nutrients it needs, leading to weight loss, fatigue, and persistent diarrhea. Some people are sensitive to gluten without having full celiac disease, experiencing milder but still disruptive symptoms.
You can also be intolerant to virtually any food or ingredient. If your diarrhea follows a pattern, showing up after specific meals, keeping a food diary for a week or two can help you spot the connection.
Medications That Cause Diarrhea
If your diarrhea started around the same time as a new prescription or supplement, that’s probably not a coincidence. Antibiotics are the most well-known offenders. They kill off beneficial gut bacteria alongside the harmful ones, disrupting the balance in your intestines. This can happen during a course of antibiotics or shortly after finishing one.
Magnesium-containing antacids draw extra water into the bowel and frequently cause loose stools. Metformin, one of the most widely prescribed diabetes medications, is another common trigger, particularly in the first few weeks of use. Laxatives, certain blood pressure medications, and some anti-inflammatory drugs can also be responsible. If you suspect a medication, don’t stop taking it on your own, but it’s worth a conversation about alternatives or timing adjustments.
Chronic Conditions Behind Ongoing Diarrhea
When diarrhea persists beyond four weeks, the American College of Gastroenterology classifies it as chronic. At that point, a one-off stomach bug is unlikely to be the explanation, and an underlying condition becomes more probable.
Irritable bowel syndrome (IBS) is one of the most common chronic causes. It produces stomach pain and changes in bowel habits, sometimes alternating between diarrhea and constipation. The key distinction is that IBS does not cause inflammation or physical damage to your digestive tract. It’s a disorder of gut function, not structure, which means standard tests often come back normal even though your symptoms are very real.
Inflammatory bowel disease (IBD), which includes Crohn’s disease and ulcerative colitis, is a different situation entirely. IBD is an autoimmune condition that causes physical damage to the digestive tract, along with inflammation, bleeding, and nutrient malabsorption. The symptoms can overlap with IBS, including stomach pain and diarrhea, but IBD tends to also involve bloody stool, weight loss, and fatigue. Both celiac disease and IBD can prevent your body from getting the nutrients it needs, so unexplained weight loss or nutritional deficiencies alongside chronic diarrhea are important signals.
How Duration Helps You Narrow It Down
The timeline of your diarrhea is one of the most useful clues. Acute diarrhea lasts less than two weeks and is most often caused by a virus, a bout of food poisoning, or a medication reaction. Persistent diarrhea, lasting two to four weeks, may point to a parasite like Giardia or an ongoing dietary trigger you haven’t removed. Chronic diarrhea, lasting more than four weeks, raises the possibility of conditions like IBS, IBD, celiac disease, or other digestive disorders.
Pay attention to what accompanies the diarrhea, too. Cramping that improves after a bowel movement is more typical of IBS. Bloody stool and fever point toward a bacterial infection or IBD. Diarrhea that reliably follows meals containing dairy or bread suggests a food intolerance. Diarrhea that started within days of a new medication speaks for itself.
Dehydration: The Real Short-Term Risk
The biggest immediate danger from diarrhea isn’t the diarrhea itself. It’s the fluid and electrolyte loss that comes with it. In adults, early signs of dehydration include dark-colored urine, sunken eyes or cheeks, and skin that doesn’t flatten back right away after being pinched. Excessive thirst, dizziness, and weakness are signals that you’re already significantly dehydrated.
Children dehydrate faster and show different warning signs: a dry mouth, no tears when crying, sunken eyes, and skin that stays tented after a pinch. In infants, a sunken soft spot on top of the skull is a serious sign. A child whose diarrhea hasn’t improved within 24 hours, who hasn’t had a wet diaper in three or more hours, or who seems unusually sleepy or unresponsive needs medical attention quickly.
For most adults, sipping water, broth, or an oral rehydration solution steadily throughout the day is enough to stay ahead of the fluid loss. Avoid caffeine and alcohol, which pull more water out of your system.
Signs That Need Medical Attention
Most diarrhea resolves without treatment, but certain symptoms change the picture. Bloody or black stools suggest bleeding somewhere in the digestive tract. A fever above 102°F (39°C) points toward a more serious infection. Severe abdominal or rectal pain, signs of dehydration that aren’t improving with fluids, and diarrhea that lasts more than two days without any improvement in adults all warrant a medical evaluation. For adults, the combination of persistent diarrhea with little or no urination and dark urine is a clear signal that your body is losing more fluid than you’re replacing.