Root canal therapy (RCT) is needed when the blood or nerve supply of a tooth (known as the pulp) becomes injured through decay or trauma (including repeated drilling).There are phases through which the pulp deteriorates and if left untreated the affected tooth will develop an abscess on its root-tip. In the acute phase, an abscess may cause a swelling in the mouth.

There is a full range of pain associated. In the early stages of nerve deterioration, hot and cold stimuli will usually cause an intense wave of pain that lingers. The next phase leads to spontaneous onset of discomfort from the tooth without any stimulus, rhyme or reason. When the nerve dies there is no pain, but once time moves on and the abscess forms a pressure builds up making the tooth sore to put pressure on.