Diarrhea can be either short-term (acute) or long-term (chronic) in duration. Almost everyone experiences the severe and sudden development of acute diarrhea at some point in their lives. It is usually short-lived (a few days) and may run its course without the need for medication. Acute diarrhea is most often caused by infection (infectious diarrhea), and may also be a side effect of many different prescription or over-the-counter medications.

Chronic diarrhea, on the other hand, lasts for at least four weeks and may indicate a more serious problem that requires treatment. Irritable bowel syndrome (IBS) is the most common cause of chronic diarrhea in the United States.

Diarrhea can be categorized into different types, depending on the processes that drive it. For example, the rapid transit of stool through the digestive tract is responsible for diarrhea that occurs with motility disorders such as IBS. Food that passes too quickly through the intestines or in too large an amount to be properly absorbed is a common cause of diarrhea. Additional types of diarrhea by process include:

• Osmotic diarrhea. Occurs when sugars (e.g., lactose, fructose), sugar alcohols (e.g., sorbitol) and some minerals (e.g., magnesium, sulfate, phosphate) in the intestines fail to be absorbed into the body. These unabsorbed substances draw water into the intestines, causing watery stool. Diarrhea caused by lactose intolerance is an example of osmotic diarrhea. Sorbitol is also a common cause of this type of diarrhea. Typically, osmotic diarrhea goes away when foods containing these elements are removed from a patient’s diet.
  
  
• Secretory diarrhea. Occurs when the intestines release water and salt into the stool, making it runny. This can be due to numerous causes, the most common being infections that release toxins. The toxic substances interfere with the intestines, causing them to secrete water and salt. Carcinoid tumors can also stimulate secretions that lead to secretory diarrhea. Secretory diarrhea can be due to poor absorption of bile acids and fatty acids by the intestines. This may occur in people with certain medical conditions, such as celiac disease and inflammatory bowel disease (IBD).
  
  Patients with a digestive system that moves digestive products too quickly are also prone to secretory diarrhea, because the intestines do not have enough time to absorb fluid. Secretory diarrhea usually continues even if a person fasts.
  
  
• Exudative diarrhea. When inflammation or irritation of the lining of the colon causes the release of blood, mucus or other fluids. This type of diarrhea can be caused by a number of conditions (e.g., IBD, tuberculosis, some cancers).  

It is important to note that different types of diarrhea may occur at the same time. For example, it is rare that patients who see a physician about their diarrhea have solely osmotic or solely secretory diarrhea. Some viral or inflammatory conditions can cause both to occur simultaneously.

Paradoxical diarrhea is the appearance of diarrhea-like symptoms when the cause is actually a complication of constipation called a fecal impaction. In patients with paradoxical diarrhea, liquid stool flows around hard, impacted stool still inside the rectum, giving the false impression of diarrhea. Paradoxical diarrhea most commonly occurs in children, the elderly or adults with limited mobility or mental capacity.