What Causes Diarrhea?

There Are Many Causes, But Diarrhea Usually Resolves On Its Own

A transmission electron micrograph (TEM) of rotavirus, which is a common cause of diarrhea.. Image © BSIP / UIG / Getty Images

Diarrhea is a common problem that affects healthy adults a few times a year. In some cases, the cause of diarrhea may be attributable to a specific cause, but many times a case of diarrhea comes and goes without knowing why.

A simple case of diarrhea is usually not a cause for alarm. However, if diarrhea goes on for more than a few days or is accompanied by abdominal pain, vomiting, blood in the stool or vomit, or other unusual symptoms, seek medical attention.

There are several potential reasons for diarrhea, including:

  • Bacterial infections
  • Food intolerances
  • Functional bowel disorders
  • Intestinal diseases
  • Medications
  • Parasites
  • Recent galbladder removal surgery
  • Viral infections

This article will discuss each of these causes for loose stools.

What Causes Diarrhea?

Diarrhea is defined as three or more loose stools a day. As long as it resolves, and goes away on its own, there is usually no reason to see a doctor or to try to stop it with anti-diarrheal agents. It can be a concern when diarrhea causes too much fluid loss, resulting in dehydration. If dehydration occurs in the very young or the very old it can be dangerous, so it's important to seek medical attention in these instances.

  • Adverse effects of medication. A side effect of some medications, which are far too numerous to list, could be diarrhea. Some of the more common medication classes that can cause diarrhea are non-steroidal anti-inflammatory drugs (NSAIDs), antibiotics, blood pressure medications, and antacids with magnesium.
  • Bacterial infections. While there are many kinds of beneficial bacteria in our bodies, there are many that live both inside and outside our bodies that can do harm. Some harmful bacteria can contaminate food or water, and when they are consumed, the result could be diarrhea or other symptoms. Some of the species of bacteria that can cause diarrhea include Campylobacter, Escherichia coli, Salmonella, and Shigella.
  • Food intolerances. One example of a common food intolerance is lactose intolerance. Some people are unable to digest lactose, the sugar found in milk. This can result in diarrhea, among other symptoms such as nausea and bloating. Food intolerances are highly individualized, and most people discover them by trial and error.
  • Functional bowel disorders. A functional bowel disorder is a condition where there are symptoms, but there is no evidence of inflammation or organic disease. The most common functional disorder is irritable bowel syndrome, which is poorly understood but is believed to be the result of the colon being sensitive to certain stimuli.
  • Intestinal diseases. Probably some of the most well-known causes of chronic diarrhea, diseases such as inflammatory bowel disease (IBD) and celiac disease often cause diarrhea.
  • Parasites. Parasites, while not very common in the Western world, can cause diarrhea. Eating or drinking food or water containing parasites can lead to diarrhea and other symptoms. Some of the specieis of parasites that can cause diarrhea include Giardia lamblia, Entamoeba histolytica, and Cryptosporidium.
  • Surgery. Diarrhea could occur after surgery on a part of the digestive system. One of the most common surgeries done today is the removal of the gallbladder, and diarrhea is common afterwards. Surgery on the stomach or intestines could also cause diarrhea.
  • Viral infections. In the same way that bacteria can infect the body and cause diarrhea, viruses can do the same. Sometimes the viruses that cause gastroenteritis are spread from person to person, and become a problem for people in close living quarters. Some of the viruses that can cause diarrhea are cytomegalovirus, herpes simplex virus, Norwalk virus, rotavirus, and viral hepatitis.

What To Do When Diarrhea is A Problem

Diarrhea is a common, and there are many causes. As long as the diarrhea doesn't result in dehydration and it goes away on it's own, it is probably nothing to worry about. However, if it goes on for several days without relief, it is time to seek help from a physician. Diarrhea that comes and goes over the course of several weeks or months is also a reason to seek care, because some of the causes for diarrhea (such as functional bowel disorders or bowel diseases) can cause symptoms that come and go. Ask the advice of a physician if diarrhea is accompanied by vomiting, abdominal pain, dehydration, blood in the stool, or if it occurs after travel to a developing country.


Guerrant RL, Van Gilder T, Steiner TS, et al; Infectious Diseases Society of America. "Practice guidelines for the management of infectious diarrhea." Clin Infect Dis. 2001 Feb 1;32(3):331-351.

Ochoa B, Surawicz CM. "Diarrheal Diseases – Acute and Chronic." American College of Gastroenterology. Apr 2007.

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