The exact cause of irritable bowel syndrome (IBS) is not known. However, health experts believe faulty communication between the brain and the intestinal tract causes the symptoms of IBS.

A complex combination of elements, including psychological stress, hormones, the immune system, and chemicals called neurotransmitters, appears to interfere with messages between the brain and the bowel. The miscommunication causes abnormal muscle contractions or spasms, which often cause cramping pain. The spasms may either speed the passage of stool, causing diarrhea, or slow it down, causing constipation or bloating.

People who have IBS seem to have unusually sensitive intestines. It is not known why their intestines are more likely to react strongly to the elements that contribute to IBS. People who have IBS may start having symptoms because of one or more factors, including:

• Eating (though no particular foods have been associated with IBS).
• Stress. Stress may affect the movements of the intestines and also may affect the way a person feels pain. (Stress may also have the same effect on people who do not have IBS.)
• Trapped gas that causes bloating.
• Hormonal changes, such as during the menstrual cycle.
• Some medicines, such as antibiotics.
• Genetics. IBS may be more likely to occur in people who have a family history of the disorder.