An anal fissure is a tear in the lining of the lower rectum (anal canal) that causes pain during bowel You are constipated and try to pass a hard stool. You have. An anal fissure is a small tear in the skin overlying the anus that may occur when a hard stool is passed. Constipation is the most common cause of anal fissures. An anal fissure (also called fissure-in-ano) is a small rip or tear in the lining of the Narcotic pain medications are avoided because they can cause constipation.