What causes diarrhea?

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Lawrence S. Friedman, MD
Gastroenterology
Diarrhea may be caused by viruses, bacteria, or parasites, as well as by various foods, drugs, and medical conditions or treatments.

Viruses. A wide variety of viruses can cause diarrhea (viral gastroenteritis). Among them are rhinovirus or adenovirus, rotavirus (the most common cause of diarrhea in infants), influenza, and norovirus (the most common cause in adults). Most diarrhea is not caused by viruses, although many of the most severe cases are.

Bacteria. A number of bacteria are associated with diarrhea. Shigella, Vibrio cholerae, Escherichia coli, and Clostridium difficile produce toxins that cause diarrhea, while salmonella and campylobacter invade the lining of the intestines and produce inflammation and diarrhea.

Parasites. Intestinal parasites, such as Giardia intestinalis, Cryptosporidium parvum, and roundworms or tapeworms, may cause diarrhea. These parasites are often found in untreated or contaminated water. Drinking untreated water from a lake or stream while camping is a common way to pick up Giardia parasites.

Diseases of the bowel. Crohn's disease and ulcerative colitis, two forms of inflammatory bowel disease, can cause diarrhea.

Immune deficiency. Patients suffering from diseases such as AIDS or those who are undergoing treatments that weaken the immune system and damage the lining of the intestine, such as chemotherapy, may also suffer from severe diarrhea.

Stress. Emotions are known to wreak havoc on the bowels in a number of ways. Diarrhea is a common complaint of persons under severe stress or emotional upset.

Foods. Certain foods, even if perfectly fresh, can cause diarrhea in some people. Among them are fruits, beans, and coffee. For most people, unripe fruits or any type of spoiled food will cause diarrhea, as will the particular foods that a person cannot tolerate, such as milk products for those who are lactose intolerant.

Medications. A number of drugs, both prescription and over the counter, can cause diarrhea as a side effect. The most common culprits include antibiotics, antacids containing magnesium, and some blood pressure and heart medications. Because antibiotics kill some of the naturally occurring gastrointestinal (GI) bacteria, the gut becomes more vulnerable to attack by Clostridium difficile, a bacterium that produces toxins that can cause diarrhea.
Michael T. Murray, ND
Naturopathic Medicine
Diarrhea is divided into four major types: osmotic, secretory, exudative, and inadequate contact.
  • Osmotic diarrhea can be the result of carbohydrate malabsorption (e.g., lactose intolerance); magnesium salts; and excess vitamin C intake.
  • Secretory diarrhea can be the result of toxin producing bacteria; hormone-producing tumors; fat malabsorption (e.g., lack of bile output); laxative abuse; and surgical resection of the small intestine.
  • Exudative diarrhea can be caused by inflammatory bowel disease (Crohn's disease or ulcerative colitis); pseudomembranous colitis (a postantibiotic diarrhea caused by an overgrowth of the bacterium Clostridium difficile); and bacterial infection by other organisms. 
  • Inadequate contact diarrhea is the result of surgical removal of sections of the intestine.
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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.