Q. A few months ago, you wrote about overgrowth of bacteria in the intestines as a cause of chronic diarrhea. I'd like to know what else could cause chronic diarrhea. My situation doesn't seem to be related to overgrowth of bacteria.

A. When dealing with a patient who has diarrhea, doctors often first decide whether it's something that's acute--lasting a few days to a week or so--or chronic--lasting weeks or months. That's because the causes of these two types of diarrhea are usually different.

For example, a common cause of acute diarrhea is an infection of the intestines. Chronic diarrhea is more likely to be caused by an alteration of normal bowel function, as in the case of irritable bowel syndrome.

Dozens of conditions can lead to chronic diarrhea, so it can take some detective work to track down the cause. Your doctor may not have many clues to go on, and in most cases there are no tests that will readily pinpoint the problem.

Sometimes the type of diarrhea provides a clue. If there's blood or white blood cells in a stool sample, that can signal inflammation. Common causes of inflammation are ulcerative colitis and Crohn's disease. These are inflammatory bowel diseases that can cause fever, abdominal pain and problems outside the intestines, such as arthritis.

Less common causes of chronic diarrhea include infections with parasites such as Giardia and ameba. People who have AIDS sometimes get unusual infections of the intestines that can result in chronic diarrhea. In addition, diarrhea is a side effect of many medicines used to treat HIV, the human immunodeficiency virus that causes AIDS.

Another type of chronic diarrhea occurs when your intestines are unable to digest or absorb something in the foods you eat. For example, people who have lactose intolerance cannot absorb the milk sugar known as lactose. The lactose passes through the intestines, pulling water along with it, resulting in loose bowel movements and abdominal cramps.

People who have had chronic pancreatitis may not be able to absorb fats and other nutrients. This problem is known as malabsorption. If your pancreas isn't working properly, the fats may pass through your intestines, resulting in greasy, foul-smelling stools. Malabsorption also occurs in people with celiac disease, an allergy to gluten, which is a protein found in wheat.

If you've had your gallbladder out, you might develop some diarrhea. This results from the digestive juices known as bile salts passing through your intestines. These juices were stored in your gallbladder until you needed them after a meal. Without a gallbladder, they pass through the intestines, sometimes leading to diarrhea.

As I've mentioned, irritable bowel syndrome is a common cause of chronic diarrhea. It can also cause constipation or alternating bouts of diarrhea and constipation. The cause of irritable bowel syndrome isn't known, but it changes the way the intestines function. Besides causing diarrhea and constipation, it also leads to abdominal cramps, gas and mucus. There's no specific test for the condition. Your doctor diagnoses it by considering the combination of symptoms and ruling out other causes.

Many years of diabetes can damage the nerves that control the intestines. In time, this can lead to diarrhea and even leakage of watery stool. Oddly, people with chronic constipation can develop a form of diarrhea known as overflow diarrhea. This occurs when liquid stool leaks around an area of blockage within the intestines caused by long-standing constipation.

Certain drugs can trigger diarrhea, such as antacids that contain magnesium; antibiotics (by changing the bacteria normally present in the intestines); Cytotec (which protects against stomach ulcers); Depakote (prescribed for seizures); Glucophage (for diabetes); potassium replacement pills; quinidine (for irregular heartbeats); and Zoloft (for depression). You can also develop diarrhea if you eat a lot of candy, gum or diet foods sweetened with sorbitol, which the body doesn't absorb well.

To figure out what might be going on, your doctor will do a history and physical examination. You will probably have some blood tests and tests of a stool sample. You might have an endoscopic procedure to examine your intestines from the inside, along with a biopsy of your intestines. In addition, you might have one or more X-ray tests to check your intestines.

Treatment for chronic diarrhea will vary depending on the cause, so you shouldn't simply take an anti-diarrhea medicine without getting a thorough checkup.

Jay Siwek, chairman of the department of family medicine at Georgetown University Medical Center, practices at the Fort Lincoln Family Medicine Center and Providence Hospital in Northeast Washington.

Consultation is a health education column and is not a substitute for medical advice from your physician. Send questions to Consultation, Health Section, The Washington Post, 1150 15th St. NW, Washington, DC 20071. Questions cannot be answered personally.