Diarrhea is among the most common reasons adults visit a primary care clinic or urgent care center, yet its causes span a remarkably wide range — from a 24-hour viral illness that resolves on its own to a sign of inflammatory bowel disease, colorectal cancer, or a serious infection requiring specific antibiotic treatment. Understanding what causes diarrhea and recognizing when diarrhea may be serious is essential for knowing when home management is appropriate and when medical evaluation is needed.
Globally, diarrheal illness causes an estimated 1.7 billion cases per year and remains a leading cause of death in children under five in low-income countries. In adults in developed countries, most acute diarrhea is self-limiting and resolves within a few days. Chronic diarrhea — lasting more than 30 days — affects an estimated five to seven percent of the general adult population and requires a more systematic evaluation to identify the underlying cause.
How Diarrhea Is Classified — Acute, Persistent, and Chronic
Diarrhea is formally defined as three or more loose or watery stools per day, or a stool output exceeding 200 grams per day. The Bristol Stool Chart types 6 (fluffy pieces with ragged edges) and 7 (entirely liquid, no solid pieces) correspond to loose and watery stool, respectively.
Duration is the most clinically useful first classification:
- Acute diarrhea lasts fewer than 14 days. The vast majority of acute diarrhea in adults is infectious in origin — viral, bacterial, or parasitic — and resolves without specific treatment other than supportive care and rehydration.
- Persistent diarrhea lasts 14 to 29 days. This category often represents a prolonged infectious cause (such as Giardia or post-infectious IBS) or an early presentation of an underlying chronic condition.
- Chronic diarrhea lasts 30 days or more. Chronic diarrhea is rarely infectious and almost always has a functional (such as IBS), inflammatory (IBD, microscopic colitis), malabsorptive (celiac disease, pancreatic insufficiency), or drug-related cause.
Stool characteristics also guide classification. Large-volume watery diarrhea without blood or mucus suggests a small bowel or secretory mechanism. Small-volume frequent stools with urgency, blood, and mucus suggest a colonic and often inflammatory cause. Greasy, pale, floating, foul-smelling stools that are difficult to flush suggest fat malabsorption (steatorrhea), which points to pancreatic or small bowel disease. Nocturnal diarrhea — diarrhea that wakes a patient from sleep — is an important sign of organic disease, since functional disorders (such as IBS) rarely cause symptoms at night.
Mechanisms — Why Diarrhea Happens
Understanding the mechanism of diarrhea helps explain why different causes produce different patterns and respond to different treatments.
Osmotic diarrhea occurs when non-absorbable solutes in the gut draw water into the intestinal lumen through osmosis. Common examples include lactose malabsorption (undigested lactose is fermented by colonic bacteria, drawing water and producing gas), fructose malabsorption, sorbitol-containing sugar-free products, and magnesium-containing antacids or supplements. A key feature of osmotic diarrhea is that it stops when the patient fasts, because the responsible solute is no longer being ingested.
Secretory diarrhea occurs when the intestinal epithelium actively secretes chloride and water into the gut lumen, exceeding absorptive capacity. Classic causes include cholera toxin, enterotoxigenic E. coli (ETEC) toxin, VIPomas (rare neuroendocrine tumors secreting vasoactive intestinal peptide), and bile acid malabsorption. Unlike osmotic diarrhea, secretory diarrhea characteristically persists during fasting, as the secretory process is independent of food intake.
Inflammatory (exudative) diarrhea results from mucosal damage — infection, inflammation, or malignancy disrupts the integrity of the intestinal lining, allowing blood, protein, and pus to enter the stool. Inflammatory bowel disease (Crohn’s disease and ulcerative colitis), infectious colitis (Salmonella, Campylobacter, Clostridioides difficile), and colorectal cancer all operate through this mechanism. Blood and mucus in the stool are the clinical hallmarks.
Motility-related diarrhea results from reduced transit time — stool moves through the colon so rapidly that insufficient water is absorbed. This is the predominant mechanism in IBS with diarrhea (IBS-D) and in hyperthyroidism, though it also contributes to post-surgical diarrhea after colon resection.
Malabsorptive diarrhea occurs when the small intestine fails to adequately absorb fat, protein, or carbohydrates — due to mucosal disease (celiac, Crohn’s), pancreatic enzyme deficiency, bacterial overgrowth, or short bowel syndrome. The fat malabsorption variant (steatorrhea) produces the characteristic greasy, foul-smelling, pale floating stool described above.
Acute Diarrhea — Viral, Bacterial, and Parasitic Causes
Most acute diarrhea in immunocompetent adults in developed countries is caused by viral gastroenteritis and resolves within 24 to 72 hours without specific treatment. The most common causative viruses are norovirus (responsible for a large proportion of food-borne illness outbreaks) and rotavirus (predominantly pediatric but also affecting adults). Adenovirus, astrovirus, and sapovirus are less common. Viral gastroenteritis typically produces watery diarrhea, nausea, vomiting, and low-grade fever, often with abdominal cramping.
Bacterial causes tend to produce more severe symptoms and more commonly cause bloody diarrhea (dysentery). The most clinically important bacterial pathogens include:
- Salmonella — transmitted through undercooked poultry, eggs, and contaminated produce; causes watery to bloody diarrhea with fever; most cases resolve in 3–7 days without antibiotics, but antibiotics are indicated for severe illness or immunocompromised patients
- Campylobacter jejuni — the most common bacterial cause of food-borne diarrhea in many countries; transmitted through undercooked chicken, unpasteurized milk, and contaminated water; typically causes bloody diarrhea with significant abdominal pain and fever; treatment with azithromycin is indicated for severe or prolonged illness
- E. coli O157:H7 (Shiga toxin-producing E. coli, STEC) — associated with undercooked ground beef and contaminated produce; can cause hemorrhagic colitis and, particularly in children and the elderly, hemolytic uremic syndrome (HUS) — a potentially life-threatening complication involving kidney failure, anemia, and thrombocytopenia; antibiotics are contraindicated in STEC infection as they may increase HUS risk
- Shigella — highly infectious; spreads person-to-person with very low infectious dose; causes dysentery with bloody mucoid stools, fever, and tenesmus
Parasitic causes should be considered when diarrhea persists beyond one to two weeks, when travel to endemic regions occurred, or when community water supply contamination is suspected. Giardia lamblia is the most common intestinal parasite in the US; it typically causes watery diarrhea, bloating, excess flatulence, and a characteristic sulfurous odor to stools, persisting for weeks if untreated. Cryptosporidium is a concern particularly in immunocompromised patients (including those with HIV). Entamoeba histolytica can cause amoebic dysentery with bloody stool and, in severe cases, amoebic liver abscess.
Clostridioides difficile — A Common and Serious Cause
Clostridioides difficile (formerly Clostridium difficile, commonly called C. diff) is a toxin-producing bacterium that colonizes the colon when the normal gut microbiome is disrupted. It is a leading cause of healthcare-associated diarrhea and is increasingly common in community settings. Understanding C. diff is important because it requires specific treatment and because standard over-the-counter anti-diarrheal medications can worsen the infection.
Risk factors for C. difficile infection include recent antibiotic use (virtually any antibiotic can predispose, but fluoroquinolones, clindamycin, cephalosporins, and broad-spectrum penicillins carry the highest risk), hospitalization, age over 65, use of proton pump inhibitors, immunosuppression, and a history of prior C. difficile infection (which significantly raises the risk of recurrence).
Symptoms typically begin during or within 8 to 12 weeks of antibiotic therapy and include watery diarrhea (three or more loose stools per day), lower abdominal cramping, and fever. Severe C. difficile colitis (pseudomembranous colitis) can produce high fever, severe abdominal pain, leukocytosis, and hypotension. Toxic megacolon — life-threatening dilation of the colon with risk of perforation — is a rare but potentially fatal complication.
Diagnosis is made by testing stool for C. difficile toxin or by PCR detection of toxin genes. Treatment currently favors oral vancomycin or fidaxomicin over metronidazole, which was the former standard. Bezlotoxumab — a monoclonal antibody — can reduce recurrence risk in high-risk patients receiving antibiotic therapy. Fecal microbiota transplantation (FMT) is effective for recurrent C. difficile infection and restores a healthy microbial community to the colon.
Drug-Induced and Food-Related Diarrhea
Many commonly prescribed and over-the-counter medications cause diarrhea as a side effect. Identifying drug-related diarrhea requires correlating the onset of diarrhea with changes in medication regimen and is often a diagnosis of exclusion. Key medications include:
- Metformin — the most commonly prescribed diabetes medication causes GI side effects including diarrhea in up to 30 percent of patients; dose-related and often improves with gradual dose escalation or switching to the extended-release formulation
- Magnesium-containing supplements or antacids — osmotic mechanism; dose-dependent
- PPIs (proton pump inhibitors) — associated with microscopic colitis and with C. difficile colonization by reducing gastric acid barrier
- SSRIs and SNRIs — increase serotonin signaling in the gut, accelerating motility, particularly at initiation
- Antibiotics — disrupt gut microbiome, causing antibiotic-associated diarrhea in up to 35 percent of patients (most mild; a small proportion develop C. difficile)
- Chemotherapy agents — several chemotherapy drugs cause severe diarrhea through mucosal damage; irinotecan is particularly associated with severe delayed diarrhea
- Colchicine, NSAIDs, thyroid hormone, digoxin — various mechanisms
Food-related diarrhea can reflect true food intolerance (lactose, fructose, sorbitol) or a reaction to contaminated food. Food poisoning from preformed bacterial toxins (Staphylococcus aureus, Bacillus cereus) typically causes diarrhea and vomiting within one to six hours of eating a contaminated meal — the rapid onset distinguishes it from infection, which requires an incubation period of typically 12 to 72 hours.
Chronic Diarrhea — Functional and Structural Causes
When diarrhea persists beyond 30 days, the causes shift substantially toward functional and structural conditions rather than infections. The most common causes of chronic diarrhea in adults are:
Irritable bowel syndrome with diarrhea (IBS-D) is the single most common diagnosis in adults with chronic loose stools. IBS-D is characterized by recurrent abdominal pain associated with defecation, change in stool frequency or form, and symptoms present for at least three months. Importantly, symptoms are typically absent at night, worsen with stress, and are often triggered by specific foods (fatty meals, caffeine, alcohol). Unlike inflammatory causes, there is no structural damage to the bowel — the gut functions abnormally due to altered motility, visceral hypersensitivity, and gut-brain axis dysregulation. Our dedicated article on irritable bowel syndrome covers IBS-D in depth.
Microscopic colitis is an inflammatory condition of the colon that is invisible on standard colonoscopy (the colon appears normal macroscopically) but shows characteristic changes on mucosal biopsy: collagenous colitis (thickened subepithelial collagen band) and lymphocytic colitis (increased intraepithelial lymphocytes). It predominantly affects women in their 50s and 60s and produces high-volume, non-bloody, watery diarrhea — often 10 or more bowel movements per day. It is strongly associated with NSAID use, PPI use, and several other medications. Budesonide is the most effective treatment.
Inflammatory bowel disease — Crohn’s disease and ulcerative colitis — produces chronic diarrhea through mucosal inflammation. Ulcerative colitis classically causes bloody mucoid diarrhea with urgency and tenesmus. Crohn’s disease can affect any segment of the GI tract and causes diarrhea that varies based on disease location — small bowel Crohn’s often causes malabsorptive diarrhea without blood; colonic Crohn’s can produce bloody diarrhea indistinguishable from UC.
Celiac disease is an autoimmune condition triggered by dietary gluten (wheat, barley, rye) that damages the small intestinal villi, producing malabsorption. Classic symptoms include bulky, fatty, foul-smelling stools, weight loss, anemia, and fatigue. However, many adults with celiac disease present with non-classical or minimal symptoms — iron deficiency anemia, osteoporosis, or subtle GI complaints — and diagnosis requires serologic testing (anti-tTG IgA) confirmed by duodenal biopsy.
Our article on common digestive problems in adults covers several of these chronic conditions in context with one another.
Red Flags — When Diarrhea May Be Serious
Most acute diarrhea in otherwise healthy adults does not require medical evaluation. However, certain features should prompt same-day or urgent evaluation, regardless of suspected cause.
- Blood in the stool — visible blood or blood mixed into the stool (not just on toilet paper from hemorrhoids) indicates potential mucosal injury from infection, IBD, ischemia, or malignancy. Our article on blood in stool: possible causes and warning signs details the evaluation approach.
- Signs of dehydration — severe thirst, dark concentrated urine, dizziness when standing (orthostatic hypotension), dry mouth, confusion, or reduced urination indicate significant fluid loss that may require IV fluid replacement, particularly in elderly patients or those with heart or kidney disease.
- High fever (above 38.5°C / 101.3°F) — suggests a bacterial or severe inflammatory cause requiring medical assessment rather than self-treatment.
- Duration beyond 7 days without improvement — warrants evaluation to identify a treatable infection or underlying condition.
- Recent antibiotic use — raises suspicion for C. difficile, which should not be managed with OTC anti-diarrheal medications.
- Nocturnal diarrhea — diarrhea that awakens the patient from sleep strongly suggests organic disease (IBD, microscopic colitis, secretory diarrhea) rather than IBS.
- Diarrhea in elderly patients or immunocompromised individuals — these groups are at higher risk of dehydration, bacteremia from enteric infections, and severe complications, and have a lower threshold for evaluation and treatment.
- Unexplained weight loss alongside chronic diarrhea — raises concern for malabsorption (celiac, pancreatic insufficiency), IBD, or malignancy.
Evaluation of Diarrhea — When and What to Test
For mild acute diarrhea without red flags, no testing is required — supportive care and monitoring are appropriate. When evaluation is indicated, it is directed by the clinical picture.
Stool studies for acute diarrhea with fever or bloody stool include stool culture (Salmonella, Campylobacter, Shigella, E. coli O157), C. difficile toxin or PCR (in patients with recent antibiotics or hospitalization), fecal leukocytes or calprotectin (a marker of mucosal inflammation), and ova and parasite examination when parasitic infection is suspected. For chronic diarrhea, fecal calprotectin and lactoferrin help distinguish inflammatory from non-inflammatory causes before colonoscopy.
Blood tests relevant to diarrhea include CBC (anemia, leukocytosis), CMP (kidney function, electrolytes), TSH (hyperthyroidism), CRP and ESR (inflammatory markers), and celiac serologies (anti-tTG IgA, total IgA to exclude IgA deficiency).
Colonoscopy is the primary endoscopic evaluation for chronic diarrhea of unknown cause, providing visualization of the colon, opportunity for mucosal biopsy (essential for diagnosing microscopic colitis — which requires biopsy even when the colon looks normal), and detection of polyps or malignancy. Upper endoscopy with duodenal biopsy is indicated when celiac disease is suspected.
Treatment — Rehydration, Medications, and Dietary Approaches
The cornerstone of diarrhea management is oral rehydration. Oral rehydration solutions (ORS) — containing glucose, sodium, potassium, and water in specific ratios — exploit glucose-sodium co-transport in the small intestine to maximize fluid absorption even in the presence of active diarrhea. The World Health Organization’s ORS formula is the gold standard for moderate to severe dehydration. Commercial preparations (Pedialyte, Normalyte) are widely available. Plain water alone is inadequate for significant fluid losses because it lacks electrolytes; sports drinks are also suboptimal due to high sugar and low sodium content.
Loperamide (Imodium) reduces intestinal motility and increases water reabsorption; it is effective for acute non-inflammatory diarrhea, traveler’s diarrhea, and IBS-D. Critically, loperamide should be avoided when diarrhea is bloody, when C. difficile is suspected, or when high fever is present — in these situations, reducing motility can worsen illness by prolonging contact with toxins or organisms.
Bismuth subsalicylate (Pepto-Bismol) has mild anti-secretory and antimicrobial properties and is effective for mild traveler’s diarrhea and acute viral gastroenteritis. It should be avoided in patients taking anticoagulants or aspirin due to salicylate content.
Probiotics — particularly Lactobacillus rhamnosus GG and Saccharomyces boulardii — have modest evidence for reducing duration of acute infectious diarrhea and preventing antibiotic-associated diarrhea. They are not a substitute for appropriate medical treatment in moderate to severe cases.
Dietary approach: The traditional BRAT diet (bananas, rice, applesauce, toast) was widely recommended for acute diarrhea but modern guidelines suggest that adults who feel able should continue eating regular, easily digestible foods rather than restricting to a bland diet. High-fat foods, spicy foods, very sugary drinks, alcohol, and caffeine may worsen symptoms acutely and should be avoided. Avoiding dairy temporarily is reasonable if lactose intolerance is contributing. Our companion article on constipation and bowel regularity discusses how stool formation and dietary factors work in the other direction.
For chronic diarrhea, treatment is cause-specific: gluten-free diet for celiac disease, anti-inflammatory therapy for IBD, budesonide for microscopic colitis, low-FODMAP diet and gut-directed therapy for IBS-D, pancreatic enzyme replacement for exocrine insufficiency.
Frequently Asked Questions About Diarrhea
How long should I wait before seeing a doctor for diarrhea?
Healthy adults with mild, watery diarrhea and no red flags can safely manage at home for up to 48 to 72 hours with oral rehydration and monitoring. Seek evaluation sooner if you have blood in the stool, high fever, signs of dehydration, recent antibiotics, or if diarrhea is severe. Adults over 65, those with chronic illness, and immunocompromised individuals should have a lower threshold and contact a provider earlier.
Is diarrhea always caused by something I ate?
Not at all. While food-related causes — contamination, intolerance, or food poisoning — are common triggers, diarrhea also results from viral infections, medications, stress, travel, antibiotics disrupting gut flora, and underlying conditions such as IBS, IBD, and celiac disease. If diarrhea recurs or becomes chronic, a dietary trigger is unlikely to be the complete explanation.
What is the difference between diarrhea and loose stools?
Technically, diarrhea is defined as three or more loose or watery stools per day. Someone who has one or two slightly soft stools per day does not meet this definition, though any change from their normal pattern — particularly if new, persistent, or accompanied by other symptoms — still warrants attention. A change in bowel habits lasting more than a few weeks should be discussed with a doctor. See our article on changes in bowel habits for more context.
Can stress cause diarrhea?
Yes. The gut-brain axis connects the central nervous system with the enteric nervous system — the extensive neural network lining the GI tract. Acute stress triggers the release of corticotropin-releasing hormone (CRH) and other signaling molecules that accelerate colonic motility. This is why diarrhea is common before stressful events. Chronic stress is a recognized trigger for IBS symptom flares and can alter gut microbiome composition over time, affecting long-term digestive health.
Should I avoid all dairy products when I have diarrhea?
Temporarily reducing dairy is reasonable during an acute bout of diarrhea, because intestinal inflammation can transiently reduce lactase enzyme activity — even in people who are not ordinarily lactose intolerant. This typically resolves as the GI tract recovers. For individuals with true lactose intolerance (where symptoms from dairy are a baseline issue), dairy avoidance is a longer-term dietary strategy.
Sources: National Institute of Diabetes and Digestive and Kidney Diseases — Diarrhea | American College of Gastroenterology — Diarrhea | Mayo Clinic — Diarrhea: Symptoms and Causes
Diarrhea in Specific Populations — Older Adults and Travelers
Older adults face a higher risk of complications from diarrheal illness. Reduced physiologic reserve, baseline kidney impairment, and medications such as diuretics and ACE inhibitors mean that dehydration develops more quickly and with more severe consequences. The sensation of thirst is also blunted with aging, so elderly patients may not recognize their fluid deficit until dehydration is significant. Any diarrheal illness in an adult over 70 lasting more than 24 to 48 hours warrants clinical contact, particularly if the person lives alone or has limited mobility that restricts access to fluids.
Traveler’s diarrhea affects an estimated 30 to 70 percent of international travelers depending on destination, with highest rates reported in visitors to South and Southeast Asia, sub-Saharan Africa, and Latin America. Enterotoxigenic E. coli (ETEC) accounts for the majority of cases. Preventive measures include avoiding tap water, ice, raw fruits and vegetables, and street food in high-risk areas. For high-risk travelers or those with underlying health conditions, prophylactic bismuth subsalicylate or, in some cases, a short antibiotic course may be recommended. For treatment, azithromycin is the current drug of choice for moderate to severe traveler’s diarrhea in most destinations, including areas with fluoroquinolone-resistant Campylobacter.
Adults who have recently returned from travel and develop persistent diarrhea beyond 14 days — particularly with bloating, excess gas, and floating stools — should be tested specifically for Giardia and other parasites, as these are under-tested in routine stool cultures and require specific ova-and-parasite examination.