Diarrhea happens when your intestines stop absorbing water normally and instead push fluid into your stool. This reversal can be triggered by dozens of things, from a virus you picked up to a medication you take every day. Most episodes are acute, resolving on their own in less than a week. But if your diarrhea keeps coming back or won’t go away, the cause is usually something more specific and worth identifying.
How Your Gut Produces Diarrhea
Your intestines normally absorb water and electrolytes from the food passing through them. Diarrhea is essentially that process running in reverse. There are two main ways this happens, and the distinction matters because it points to different causes.
In osmotic diarrhea, something in your gut is pulling water in. Undigested sugars are a common culprit. If you’re lactose intolerant, for example, the lactose you can’t break down draws water into your intestines, producing loose, watery stool. The hallmark of osmotic diarrhea is that it stops when you stop eating the trigger food or substance.
In secretory diarrhea, your intestinal cells are actively pumping fluid outward, even when you haven’t eaten anything unusual. This type doesn’t improve with fasting, which is one way doctors distinguish it from osmotic diarrhea. Certain infections, hormonal conditions, and toxins can flip your gut into this secretory mode, sometimes producing a high volume of watery stool.
Infections: The Most Common Acute Cause
If your diarrhea came on suddenly and you also feel feverish, nauseated, or crampy, an infection is the most likely explanation. Viruses, bacteria, and parasites can all inflame or disrupt the lining of your digestive tract.
Rotavirus is one of the most common viral causes, producing watery diarrhea, vomiting, fever, and stomach pain that typically last 3 to 8 days. Norovirus is another frequent offender, especially in outbreaks linked to contaminated food or close quarters. Most viral diarrhea resolves on its own without treatment.
Bacterial infections tend to be more intense. Salmonella causes diarrhea, fever, and stomach cramps that begin 12 to 72 hours after exposure, often from undercooked poultry or eggs. E. coli infections can produce bloody diarrhea with severe cramping and vomiting. These bacterial cases sometimes need medical attention, particularly if you develop a high fever or see blood in your stool.
Medications That Cause Diarrhea
If your diarrhea started around the same time as a new prescription or supplement, the medication itself may be the problem. This is one of the most overlooked causes.
Antibiotics are a top offender. They work by killing bacteria, but they don’t distinguish between harmful bacteria and the beneficial ones living in your gut. When antibiotics wipe out your normal intestinal flora, opportunistic bacteria can overgrow. In some cases, this allows a particularly dangerous strain called C. difficile to take hold, which can cause severe, watery, and sometimes bloody diarrhea.
Other common medications that trigger diarrhea include:
- Metformin, widely prescribed for type 2 diabetes
- Magnesium-containing antacids, which draw water into the intestines
- NSAIDs like ibuprofen and naproxen, used for pain and inflammation
- Proton pump inhibitors and other heartburn medications
- Chemotherapy drugs
- Herbal teas containing senna or other natural laxative compounds
Laxatives, by design, cause diarrhea. They either draw water into the gut or stimulate the intestinal muscles to contract more forcefully. If you’re using them regularly, even “gentle” or “natural” ones, they can easily overshoot.
Food Intolerances and Diet
Recurring diarrhea that follows meals but doesn’t come with fever or severe illness often points to a food intolerance. Lactose intolerance is the most common example. When your body lacks enough of the enzyme that breaks down milk sugar, the undigested lactose pulls water into your intestines. The result is bloating, gas, and diarrhea within a few hours of consuming dairy.
Fructose, found in fruit juices, honey, and many processed foods, causes similar problems in some people. Sugar alcohols like sorbitol and xylitol, common in sugar-free gum and candy, are poorly absorbed by almost everyone and can cause diarrhea even in small amounts. Celiac disease, an immune reaction to gluten, is another dietary cause that produces chronic diarrhea along with nutrient malabsorption and weight loss.
If you notice a pattern between certain foods and loose stools, keeping a food diary for a week or two can help you and your doctor narrow down the trigger.
IBS vs. IBD: Two Very Different Conditions
When diarrhea becomes a recurring pattern lasting weeks or months, two conditions come up frequently: irritable bowel syndrome (IBS) and inflammatory bowel disease (IBD). They sound similar but are fundamentally different.
IBS is classified as a syndrome, meaning it’s defined by a group of symptoms rather than visible damage to your intestines. A colonoscopy in someone with IBS looks completely normal. The diagnosis is based on having abdominal pain or discomfort 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 the consistency of your stool. IBS does not cause inflammation, does not increase your risk of colon cancer, and rarely requires surgery or hospitalization.
IBD, which includes Crohn’s disease and ulcerative colitis, is an actual disease that causes destructive inflammation visible on imaging and during endoscopy. It can permanently damage the intestines and carries an increased risk of colon cancer. Symptoms that distinguish IBD from IBS include anemia, rectal bleeding, unexplained weight loss, and fever. If you’re experiencing any of those alongside chronic diarrhea, that’s a meaningful signal.
How Long Your Diarrhea Lasts Matters
The timeline of your diarrhea is one of the most useful clues to its cause. Acute diarrhea, the most common type, lasts less than a week and is usually caused by an infection or something you ate. Persistent diarrhea lasts between 2 and 4 weeks, which may indicate a lingering infection or a reaction to a medication. Chronic diarrhea, defined as lasting 4 weeks or longer, typically signals an underlying condition like IBS, IBD, celiac disease, or a hormonal disorder.
Acute episodes rarely need investigation beyond basic self-care and staying hydrated. But diarrhea that crosses the 2-week mark, or that keeps returning in cycles, usually warrants a conversation with your doctor about testing.
Signs That Need Prompt Attention
Most diarrhea is uncomfortable but not dangerous. However, certain symptoms alongside diarrhea suggest something more serious is happening. These include black or tarry stools, blood or pus in your stool, severe abdominal or rectal pain, a high fever, frequent vomiting, and signs of dehydration like dizziness, dark urine, or unusual fatigue and irritability.
In adults, diarrhea lasting more than 2 days or producing six or more loose stools per day also warrants medical evaluation. For children, the threshold is lower: diarrhea lasting more than a day, any fever in infants, or refusal to eat or drink for more than a few hours should prompt a call to their pediatrician. Dehydration develops faster in young children and older adults, making timely fluid replacement especially important in those groups.