Merck Manual

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Jonathan Gotfried

, MD, Lewis Katz School of Medicine at Temple University

Reviewed/Revised Jan 2022
Topic Resources

Stool is 60 to 90% water. In Western society, stool amount is 100 to 200 g/day in healthy adults and 10 g/kg/day in infants, depending on the amount of unabsorbable dietary material (mainly carbohydrates). Diarrhea is defined as stool weight > 200 g/day. However, many people consider any increased stool fluidity to be diarrhea. Alternatively, many people who ingest fiber have bulkier but formed stools but do not consider themselves to have diarrhea.

Frequent passage of small volumes of stool, as may occur in patients with tenesmus (rectal urgency), should be distinguished from diarrhea. Similarly, fecal incontinence Fecal Incontinence Fecal incontinence is involuntary defecation. Diagnosis is clinical. Treatment is a bowel management program and perineal exercises, but sometimes colostomy is needed. (See also Evaluation of... read more can be confused with diarrhea. However, diarrhea can cause a marked worsening of fecal incontinence.

Complications of diarrhea

Complications may result from diarrhea of any etiology. Fluid loss with consequent dehydration, electrolyte loss (sodium, potassium, magnesium, chloride), and even vascular collapse sometimes occur. Collapse can develop rapidly in patients who have severe diarrhea (eg, patients with cholera) or are very young, very old, or debilitated. Bicarbonate loss can cause metabolic acidosis Metabolic Acidosis Metabolic acidosis is primary reduction in bicarbonate (HCO3), typically with compensatory reduction in carbon dioxide partial pressure (Pco2); pH may be markedly low or slightly... read more . Hypokalemia Hypokalemia Hypokalemia is serum potassium concentration < 3.5 mEq/L (< 3.5 mmol/L) caused by a deficit in total body potassium stores or abnormal movement of potassium into cells. The most common... read more can occur when patients have severe or chronic diarrhea or if the stool contains excess mucus. Hypomagnesemia Hypomagnesemia Hypomagnesemia is serum magnesium concentration < 1.8 mg/dL (< 0.70 mmol/L). Causes include inadequate magnesium intake and absorption or increased excretion due to hypercalcemia or medications... read more after prolonged diarrhea can cause tetany.

Etiology of Diarrhea

Normally, the small intestine and colon absorb 99% of fluid resulting from oral intake and gastrointestinal (GI) tract secretions—a total fluid load of about 9 of 10 L daily. Thus, even small reductions (ie, 1%) in intestinal water absorption or increases in secretion can increase water content enough to cause diarrhea.

There are a number of causes of diarrhea ( see Table: Some Causes of Diarrhea* Some Causes of Diarrhea* Some Causes of Diarrhea* ). Several basic mechanisms cause most clinically significant diarrheas. The three most common are increased osmotic load, increased secretions/decreased absorption, and decreased contact time/surface area. In many disorders, more than one mechanism is active. For example, diarrhea in inflammatory bowel disease Overview of Inflammatory Bowel Disease Inflammatory bowel disease (IBD), which includes Crohn disease and ulcerative colitis, is a relapsing and remitting condition characterized by chronic inflammation at various sites in the gastrointestinal... read more results from mucosal inflammation, exudation into the lumen, and from multiple secretagogues and bacterial toxins that affect enterocyte function.

Osmotic load

Diarrhea occurs when unabsorbable, water-soluble solutes remain in the bowel and retain water. Such solutes include polyethylene glycol, magnesium salts (hydroxide and sulfate), and sodium phosphate, which are used as laxatives. Osmotic diarrhea occurs with sugar intolerance (eg, lactose intolerance Carbohydrate Intolerance Carbohydrate intolerance is the inability to digest certain carbohydrates due to a lack of one or more intestinal enzymes. Symptoms include diarrhea, abdominal distention, and flatulence. Diagnosis... read more caused by lactase deficiency). Ingesting large amounts of hexitols (eg, sorbitol, mannitol, xylitol) or high fructose corn syrups, which are used as sugar substitutes in candy, gum, and fruit juices, causes osmotic diarrhea because hexitols are poorly absorbed. Lactulose, which is used as a laxative, causes diarrhea by a similar mechanism. Overingesting certain foodstuffs ( see Table: Some Causes of Diarrhea* Some Causes of Diarrhea* Some Causes of Diarrhea* ) can cause osmotic diarrhea.

Increased secretions/decreased absorption

Diarrhea occurs when the bowels secrete more electrolytes and water than they absorb. Causes of increased secretions include infections, unabsorbed fats, certain drugs, and various intrinsic and extrinsic secretagogues.

Infections (eg, gastroenteritis Overview of Gastroenteritis Gastroenteritis is inflammation of the lining of the stomach and small and large intestines. Most cases are infectious, although gastroenteritis may occur after ingestion of drugs, medications... read more ) are the most common causes of secretory diarrhea. Infections combined with food poisoning are the most common causes of acute diarrhea (< 4 days in duration). Most enterotoxins block sodium-potassium exchange, which is an important driving force for fluid absorption in the small bowel and colon.

Drugs may stimulate intestinal secretions directly (eg, quinidine, quinine, colchicine, anthraquinone cathartics, castor oil, prostaglandins) or indirectly by impairing fat absorption (eg, orlistat).

Various endocrine tumors produce secretagogues, including vipomas Vipoma A vipoma is a non-beta pancreatic islet cell tumor secreting vasoactive intestinal peptide (VIP), resulting in a syndrome of watery diarrhea, hypokalemia, and achlorhydria (WDHA syndrome). Diagnosis... read more (vasoactive intestinal peptide), gastrinomas Gastrinoma A gastrinoma is a gastrin-producing tumor usually located in the pancreas or the duodenal wall. Gastric acid hypersecretion and aggressive, refractory peptic ulceration result (Zollinger-Ellison... read more Gastrinoma (gastrin), mastocytosis Mastocytosis and Mast Cell Activation Syndrome Mastocytosis is mast cell proliferation with infiltration of skin or other tissues and organs. Mast cell activation syndrome is increased and inappropriate activation of mast cells without clonal... read more Mastocytosis and Mast Cell Activation Syndrome (histamine), medullary carcinoma of the thyroid Medullary Thyroid Carcinoma Thyroid cancer can derive from follicular cells or parafollicular C-cells. Most thyroid cancers manifest as asymptomatic nodules. Diagnosis is often by fine-needle aspiration biopsy but may... read more (calcitonin and prostaglandins), and carcinoid tumors Overview of Carcinoid Tumors Carcinoid tumors develop from neuroendocrine cells in the gastrointestinal tract (90%), pancreas, pulmonary bronchi, and rarely the genitourinary tract. More than 95% of all gastrointestinal... read more (histamine, serotonin, and polypeptides). Some of these mediators (eg, prostaglandins, serotonin, related compounds) also accelerate intestinal transit, colonic transit, or both.

Impaired absorption of bile salts, which can occur with several disorders, can cause diarrhea by stimulating water and electrolyte secretion. The stools have a green or orange color.

Reduced contact time/surface area

Stimulation of intestinal smooth muscle by drugs (eg, magnesium-containing antacids, laxatives, cholinesterase inhibitors, selective serotonin reuptake inhibitors) or humoral agents (eg, prostaglandins, serotonin) also can speed transit.


Evaluation of Diarrhea


History of present illness should determine duration and severity of diarrhea, circumstances of onset (including recent travel, food ingested, source of water), drug use (including any antibiotics within the previous 3 months), abdominal pain or vomiting, frequency and timing of bowel movements, changes in stool characteristics (eg, presence of blood, pus, or mucus; changes in color or consistency; evidence of steatorrhea), associated changes in weight or appetite, and rectal urgency or tenesmus should be noted. Simultaneous occurrence of diarrhea in close contacts should be ascertained. Physicians should ask specifically about any changes in drugs that may cause diarrhea.

Review of systems should seek symptoms suggesting possible causes, including joint pains (inflammatory bowel disease, celiac disease), flushing (carcinoid, vipoma, mastocytosis), chronic abdominal pain (irritable bowel, inflammatory bowel disease, gastrinoma), and GI bleeding (ulcerative colitis, tumor).

Past medical history should identify known risk factors for diarrhea, including inflammatory bowel disease, irritable bowel syndrome, HIV infection, and previous GI surgical procedures (eg, intestinal or gastric bypass or resection, pancreatic resection). Family and social history should query about simultaneous occurrence of diarrhea in close contacts.

Physical examination

Fluid and hydration status should be evaluated. A full examination with attention to the abdomen and a digital rectal examination for sphincter competence and occult blood testing are important.

Red flags

Certain findings raise suspicion of an organic or more serious etiology of diarrhea:

  • Blood or pus in stool

  • Fever

  • Signs of dehydration

  • Chronic diarrhea

  • Weight loss

Interpretation of findings

Acute, watery diarrhea in an otherwise healthy person is likely to be of infectious etiology, particularly when travel, possibly tainted food, or an outbreak with a point-source is involved.

Acute bloody diarrhea with or without hemodynamic instability in an otherwise healthy person suggests an enteroinvasive infection. Diverticular bleeding and ischemic colitis also manifest with acute bloody diarrhea. Recurrent bouts of bloody diarrhea in a younger person suggest inflammatory bowel disease Overview of Inflammatory Bowel Disease Inflammatory bowel disease (IBD), which includes Crohn disease and ulcerative colitis, is a relapsing and remitting condition characterized by chronic inflammation at various sites in the gastrointestinal... read more .

In the absence of laxative use, large-volume diarrhea (eg, daily stool volume > 1 L/day) strongly suggests an endocrine tumor cause in patients with normal GI anatomy. A history of oil droplets in stool, particularly if associated with weight loss, suggests malabsorption Overview of Malabsorption Malabsorption is inadequate assimilation of dietary substances due to defects in digestion, absorption, or transport. Malabsorption can affect macronutrients (eg, proteins, carbohydrates, fats)... read more .

Diarrhea that consistently follows ingestion of certain foods (eg, fats) suggests food intolerance. Recent antibiotic use should raise suspicion for antibiotic-associated diarrhea, including Clostridioides difficile colitis Clostridioides (formerly Clostridium) difficile–Induced Diarrhea Toxins produced by Clostridioides difficile strains in the gastrointestinal tract cause pseudomembranous colitis, typically after antibiotic use. Symptoms are diarrhea, sometimes bloody... read more (formerly Clostridium difficile).

Diarrhea with green or orange stools suggests impaired absorption of bile salts.

The symptoms can help identify the affected part of the bowel. Generally, in small-bowel diseases, stools are voluminous and watery or fatty. In colonic diseases, stools are frequent, sometimes small in volume, and possibly accompanied by blood, mucus, pus, and abdominal discomfort.

In irritable bowel syndrome Irritable Bowel Syndrome (IBS) Irritable bowel syndrome is characterized by recurrent abdominal discomfort or pain with at least two of the following characteristics: relation to defecation, association with a change in frequency... read more (IBS), abdominal pain is related to defecation, associated with changes in stool frequency or consistency, or both. However, these symptoms alone do not discriminate IBS from other diseases (eg, inflammatory bowel disease). Functional diarrhea is characterized by loose or watery stools with onset at least 6 months before diagnosis and present during the previous 3 months. These patients do not meet the criteria for IBS; they may have abdominal pain and/or bloating, but these are not predominant symptoms (1 Evaluation reference Stool is 60 to 90% water. In Western society, stool amount is 100 to 200 g/day in healthy adults and 10 g/kg/day in infants, depending on the amount of unabsorbable dietary material (mainly... read more ). IBS with diarrhea sometimes develops in patients after an acute enteric infection (postinfectious IBS).


Acute diarrhea (< 4 days) typically does not require testing. Exceptions are patients with signs of dehydration, bloody stool, fever, severe pain, hypotension, or toxic features—particularly those who are very young or very old. These patients should have a complete blood count and measurement of electrolytes, blood urea nitrogen, and creatinine. Stool samples should be collected for microscopy, culture, and, if antibiotics have been taken recently, C. difficile toxin assay.

Chronic diarrhea (> 4 weeks) requires evaluation, as does a shorter (1 to 3 weeks) bout of diarrhea in immunocompromised patients or those who appear significantly ill. Diagnostic evaluation should be directed by the history and physical examination when possible. If this approach does not provide a diagnosis or direction, a broader approach is needed. Initial testing should include stool for occult blood, fat (by Sudan stain or fecal elastase), electrolytes (to calculate the stool osmotic gap), and Giardia antigen or polymerase chain reaction test; complete blood count with differential; celiac serology (IgA tissue transglutaminase); thyroid-stimulating hormone (TSH) and free thyroxine (T4); and fecal calprotectin or fecal lactoferrin (to screen for inflammatory bowel disease [IBD]). The 2019 American Gastroenterological Association's guidelines on the laboratory evaluation of functional diarrhea and diarrhea-predominant IBS (IBS-D) recommend a threshold value of 50 mcg/g for fecal calprotectin or a range of 4.0 to 7.25 mcg/g for fecal lactoferrin to optimize sensitivity for IBD. Microscopic examination for ova and parasites should be done for patients with recent travel history to or recent immigration from high-risk areas. Stool tests for C. difficile should be done in patients with recent antibiotic exposure or suspected C. difficile infection. Sigmoidoscopy or colonoscopy with biopsies should follow to look for inflammatory causes.

If no diagnosis is apparent and Sudan stain or fecal elastase is positive for fat, fecal fat excretion should be measured. Additional testing, including small-bowel CT enterography (structural disease) and endoscopic small-bowel biopsy (mucosal disease), can be considered (eg, if symptoms persist or are severe such as weight loss). If evaluation still yields negative findings, assessment of pancreatic structure and function ( see Laboratory tests Laboratory tests Acute pancreatitis is acute inflammation of the pancreas (and, sometimes, adjacent tissues). The most common triggers are gallstones and alcohol intake. The severity of acute pancreatitis is... read more Laboratory tests ) should be considered for patients who have unexplained steatorrhea. Infrequently, capsule endoscopy may uncover lesions, predominantly Crohn disease or nonsteroidal anti-inflammatory drug enteropathy, not identified by other modalities.

The stool osmotic gap, which is calculated 290 2 × (stool sodium + stool potassium), indicates whether diarrhea is secretory or osmotic. An osmotic gap < 50 mEq/L indicates secretory diarrhea; a larger gap suggests osmotic diarrhea. Patients with osmotic diarrhea may have covert magnesium laxative ingestion (detectable by stool magnesium levels) or carbohydrate malabsorption (diagnosed by hydrogen breath test, lactase assay, and dietary review).

Undiagnosed secretory diarrhea requires testing (eg, plasma gastrin, calcitonin, vasoactive intestinal peptide levels, histamine, urinary 5-hydroxyindole acetic acid [5-HIAA]) for endocrine-related causes. A review for symptoms of adrenal insufficiency should be done. Surreptitious laxative abuse must be considered; it can be ruled out by a fecal laxative assay.

Evaluation reference

Treatment of Diarrhea

  • Fluid and electrolytes for dehydration

  • Possibly antidiarrheals for nonbloody diarrhea in patients without systemic toxicity

Severe diarrhea requires fluid and electrolyte replacement to correct dehydration, electrolyte imbalance, and acidosis. Parenteral fluids containing sodium chloride, potassium chloride, and glucose are generally required. Salts to counteract acidosis (sodium lactate, acetate, bicarbonate) may be indicated if serum bicarbonate is < 15 mEq/L (< 15 mmol/L). An oral glucose-electrolyte solution can be given if diarrhea is not severe and nausea and vomiting are minimal ( see Solutions Solutions Oral fluid therapy is effective, safe, convenient, and inexpensive compared with IV therapy. Oral fluid therapy is recommended by the American Academy of Pediatrics and the World Health Organization... read more ). Oral and parenteral fluids are sometimes given simultaneously when water and electrolytes must be replaced in massive amounts (eg, in cholera).

Diarrhea is a symptom. When possible, the underlying disorder should be treated, but symptomatic treatment is often necessary. Diarrhea may be decreased by oral loperamide 2 to 4 mg 3 or 4 times a day (preferably given 30 minutes before meals), diphenoxylate 2.5 to 5 mg (tablets or liquid) 3 or 4 times a day, oral codeine phosphate 15 to 30 mg 2 or 3 times a day, or paregoric (camphorated opium tincture) oral liquid 5 to 10 mL once a day to 4 times a day.

Because antidiarrheals may exacerbate C. difficile colitis or increase the likelihood of hemolytic-uremic syndrome in Shiga toxin–producing Escherichia coli infection, they should not be used in bloody diarrhea of unknown cause. Their use should be restricted to patients with watery diarrhea and no signs of systemic toxicity. However, there is little evidence to justify previous concerns about prolonging excretion of possible bacterial pathogens with antidiarrheals.

Psyllium or methylcellulose compounds provide bulk. Although usually prescribed for constipation, bulking agents given in small doses decrease the fluidity of liquid stools. Kaolin, pectin, and activated attapulgite adsorb fluid. Osmotically active dietary substances (see table ) and stimulatory drugs should be avoided.

Eluxadoline, which has mu-opioid receptor agonist and delta-opioid receptor antagonist effects, can be used to treat diarrhea-predominant IBS. The dose is 100 mg orally 2 times a day (75 mg 2 times a day if the 100-mg dose cannot be tolerated). It should not be used in patients who have had a cholecystectomy or a history of pancreatitis. Patients with diarrhea-predominant IBS may also be given the antibiotic rifaximin 550 mg orally 3 times a day for 14 days.

Key Points

  • In patients with acute diarrhea, testing is only necessary for those who have prolonged symptoms (ie, > 1 week), have red flag findings, are very young, or are very old.

  • Be cautious when using antidiarrheals if C. difficile colitis, Salmonella infection, or shigellosis is possible.

  • Postinfectious inflammatory bowel syndrome develops in 10% of patients after acute infectious enteritis.

More Information

The following English-language resource may be useful. Please note that THE MANUAL is not responsible for the content of this resource.

Drugs Mentioned In This Article

Drug Name Select Trade
GaviLax, GIALAX , GlycoLax, Healthylax, MiraLax, Visine Dry Eye Relief, Vita Health
Mag-64, Mag-G , MagGel 600, Magonate, MagOx 400 , Mag-SR , Mag-Tab SR , Magtrate, Phillips Cramp-Free, Uro-Mag
Colief, Lac-Dose , Lactaid, Lactaid Fast Act, Lactrase
Numoisyn, Saliva Substitute
Aridol, BRONCHITOL, Osmitrol , Resectisol
Acilac, Cephulac, Cholac, Chronulac, Constilac, Constulose, Enulose, Generlac, Kristalose
Quinaglute, Quinora
No brand name available
alli, Xenical
Aluvea , BP-50% Urea , BP-K50, Carmol, CEM-Urea, Cerovel, DermacinRx Urea, Epimide-50, Gord Urea, Gordons Urea, Hydro 35 , Hydro 40, Kerafoam, Kerafoam 42, Keralac, Keralac Nailstik, Keratol, Keratol Plus, Kerol, Kerol AD, Kerol ZX, Latrix, Mectalyte, Nutraplus, RE Urea 40, RE Urea 50 , Rea Lo, Remeven, RE-U40, RYNODERM , U40, U-Kera, Ultra Mide 25, Ultralytic-2, Umecta, Umecta Nail Film, URALISS, Uramaxin , Uramaxin GT, Urea, Ureacin-10, Ureacin-20, Urealac , Ureaphil, Uredeb, URE-K , Uremez-40, Ure-Na, Uresol, Utopic, Vanamide, Xurea, X-VIATE
Acetasol, Borofair, VoSoL
4-Way Saline, Adsorbonac, Altamist, Ayr Allergy & Sinus, Ayr Baby Saline, Ayr Saline Nasal, BD Posiflush Normal Saline, BD Posiflush Sterile Field Normal Saline, BD Posiflush SureScrub Normal Saline, Blairex Broncho Saline, Breathe Free Saline, Deep Sea , Entsol, HyperSal, Hyper-Sal, Hypertears, Little Remedies for Noses, Little Remedies Stuffy Nose, Monoject Sodium Chloride, Muro 128, NebuSal , Ocean, Ocean Complete, Ocean For Kids, Pediamist, PULMOSAL, Rhinaris, Rhinaris Lubricating, Saljet , Saljet Rinse, SaltAire, Sea Soft, Trichotine, Wound Wash, XYNASE, ZARBEE'S Soothing Saline Nasal Mist
Cena K , ED-K+10, Epiklor, K Plus, K Plus Care, K-10 , K-8, Kaon-CL, Kay Ciel , K-Dur, K-Lor, Klor-Con, Klor-Con M10, Klor-Con M15, Klor-Con M20, Klotrix, K-Lyte CL, K-Sol , K-Tab, Micro-K, Micro-K Extencaps, POKONZA, PROAMP, Rum-K, Slow-K, Tri-K
No brand name available
Anti-Diarrheal, Imodium A-D, Imodium A-D EZ Chews , K-Pek II, Medique Diamode
No brand name available
Fiber Therapy, GenFiber , Geri-Mucil, Hydrocil , Konsyl, Metamucil, Metamucil MultiHealth, Mucilin , Natural Fiber Laxative, Natural Fiber Therapy, Reguloid
Citrucel, Fiber Therapy, Murocel, Quality Choice Fiber Theraphy
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