The exact cause of IBS is not yet established. It is a functional disorder meaning that the way the bowel works is affected, but tests on the bowel find no specific physical abnormalities that can explain the symptoms.

Symptoms are thought to be caused by muscle contractions in the bowel wall which are generally higher in frequency and stronger for people experiencing IBS. These contractions can be most troublesome after food (especially as intolerance of specific foods such as tea, coffee or dairy products can be the trigger for the symptoms) and in stressful situations. IBS can also develop after gastroenteritis.