The reasons why IBS develop are not clear. It can occur after an infection of the intestines. This is called postinfectious IBS. There may also be other triggers.
The intestine is connected to the brain. Signals go back and forth between the bowel and brain. These signals affect bowel function and symptoms. The nerves can become more active during stress. This can cause the intestines to be more sensitive and contract more.
IBS can occur at any age. Often, it begins in the teen years or early adulthood. It is twice as common in women as in men.
About 1 in 6 people in the U.S. have symptoms of IBS. It is the most common intestinal problem that causes patients to be referred to a bowel specialist (gastroenterologist).
IBS symptoms vary from person to person and range from mild to severe. Most people have mild symptoms. You are said to have IBS when symptoms are present for at least 3 days a month for a period of 3 months or more.
The main symptoms include:
Pain and other symptoms will often be reduced or go away after a bowel movement. Symptoms may flare up when there is a change in the frequency of your bowel movements.
People with IBS may go back and forth between having constipation and diarrhea or have or mostly have one or the other.
If you have IBS with diarrhea, you will have frequent, loose, watery stools. You may have an urgent need to have a bowel movement, which may be hard to control.
If you have IBS which constipation, you will have a hard time passing stool, as well as fewer bowel movements. You may need to strain with a bowel movement and have cramps. Often, only a small amount or no stool at all will be passed.
The symptoms may get worse for a few weeks or a month, and then decrease for a while. In other cases, symptoms are present most of the time.
You may also lose your appetite if you have IBS.
Exams and Tests
There is no test to diagnose IBS. Most of the time, your doctor can diagnose IBS based on your symptoms. Eating a lactose-free diet for 2 weeks may help the doctor identify lactase deficiency.
The following tests may be done to rule out other problems:
Blood tests to see if you have celiac disease or a low blood count (anemia)
Stool cultures to check for an infection
Your doctor may recommend a colonoscopy. During this test, a flexible tube is inserted through the anus to examine the colon. You may need this test if:
Symptoms began later in life (over age 50)
You have symptoms such as weight loss or bloody stools
You have abnormal blood tests (such as a low blood count)
Other disorders that can cause similar symptoms include:
Colon cancer (cancer rarely causes typical IBS symptoms, unless symptoms such as weight loss, blood in the stools, or abnormal blood tests are also present)
George F. Longstreth, MD, Department of Gastroenterology, Kaiser Permanente Medical Care Program, San Diego, California. Also reviewed by David Zieve, MD, MHA, Bethanne Black, and the A.D.A.M. Editorial team.