What Is an Anal Fissure, and Why Is It So Painful?
An anal fissure is a split in the sensitive lining just inside the anus. It often starts when a hard or large stool stretches the tissue, although frequent diarrhoea, childbirth and local irritation can also contribute. The typical symptom is a cutting or tearing pain while passing stool. Pain may continue as a deep ache or spasm for minutes or hours, and bright-red blood may be seen on the toilet paper or on the outside of the stool.
The pain is intensified by the internal anal sphincter, a ring of muscle that works automatically. A fissure can trigger this muscle to tighten; the spasm causes more pain and reduces blood flow to the injured area, making healing more difficult. Fear of pain may then lead a person to delay using the toilet, allowing stool to become harder. Treatment aims to interrupt this pain–spasm–constipation cycle while giving the tear time to close.

