There are a number of reasons why you may develop an anal fissure. These include:straining during a bowel movement-often due to constipation; recurrent or chronic diarrhoea; Crohn's disease; sexually transmitted infection-when an infection in the skin around the anus causes a tear; being in the third trimester of pregnancy; childbirth; diabetes; human immunodeficiency virus(HIV) infection or certain medicines(eg nicorandil).
Attribution: BUPA