An anal fissure is a small tear in the thin, moist tissue that lines the anus. Common causes of anal fissure include: Passing large or hard stools Constipation and straining during bowel movements Chronic diarrhea Anal intercourse Childbirth Less common causes of anal fissures include: Crohn’s disease or another inflammatory bowel disease Anal cancer HIV Tuberculosis Syphilis Factors that may increase your risk of developing an anal fissure include: Constipation. Straining during bowel movements and passing hard stools increase the risk of tearing. Childbirth. Anal fissures are more common in women after they give birth. Crohn’s disease. This inflammatory bowel disease causes chronic inflammation of the intestinal tract, which may make the lining of the anal canal more vulnerable to tearing. Anal intercourse. Age. Anal fissures can occur at any age, but are more common in infants and middle-aged adults. Find out more about anal fissure treatme...