Root canal treatment is needed for two main reasons. The first is infection. An untreated cavity is a common cause of pulp infection. The infection destroys the enamel and dentin of the tooth until it reaches the pulp. Bacteria then infect the pulp. Antibiotics can't reach all the bacteria inside a canal. They are given to reduce the number of bacteria that spread outside the canal and into the bone. The inflammation caused by the infection reduces the blood supply to the tooth. The reduced blood supply also keeps the pulp from healing.

The second reason for a root canal is damage to the pulp that can't be fixed. A fracture in a tooth can damage the pulp. So can a less severe blow to the tooth (trauma), even if there's no visible damage right away. In time, the tooth may darken because of bleeding within the pulp.