It is a functional disorder, which means that there is a problem in how the muscles in the intestines work. New research shows that this problem is linked to the over- or underproduction of serotonin by the digestive system.

Irritable bowel syndrome is characterised by gas, abdominal pain, and diarrhoea or constipation, or both. Although the syndrome can cause considerable pain and discomfort, it does not damage the digestive tract as diseases do. Also, irritable bowel syndrome does not lead to more serious digestive diseases later.