What causes an anal fissure?
Common causes
Most anal fissures are thought to be due to passing large or hard faeces when you are constipated. The rim of the anus may stretch and tear slightly. Spasm (tightening) of the muscle around the anus (the sphincter) may play a part in causing the tear, or in slowing down the healing process. (If the muscle around the anus is tight, the blood supply to the anus is reduced. This may affect how well the tear heals.)
In about 1 in 10 cases, the fissure occurs during childbirth. Sometimes an anal fissure occurs if you have bad diarrhoea.
Anal fissures and other conditions
In a minority of cases, a fissure occurs as part of another condition. For example, as a complication of Crohn's disease or an anal herpes infection. In these cases you will have other symptoms and problems as well. These type of fissures are not dealt with further in this leaflet.
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