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It isn't clear what causes IBS. The cause may be different for different people. IBS may be caused by problems with the way signals are sent between the brain and the digestive tract. In some people, this miscommunication causes abnormal muscle contractions or spasms, which often cause cramping pain. The spasms may speed the passage of stool, causing diarrhea. Or they may slow it down, causing constipation or bloating.
People with IBS may have unusually sensitive intestines or problems with the way the muscles of the intestines move. It isn't known why their intestines are more likely to react strongly to the things that contribute to IBS.
People who have IBS may start having symptoms because of one or more causes, such as:
Eating. (But no particular foods have been linked with IBS.)
Stress and psychological issues, such as anxiety and depression.
Hormonal changes, such as during the menstrual cycle.
Some medicines, such as antibiotics.
An infection in the digestive tract, such as salmonella.
Genetics. IBS may be more likely to occur in people who have a family history of it.