Root canal treatment or endodontic therapy is often the indicated treatment when someone has a toothache. The symptoms are spontaneous pain, lasting sensitivity to cold which is then relieved by heat in the earlier stage of pulpal deterioration and lasting sensitivity to heat which is relieved by cold in later stages of pulpal degeneration. As the pulp ("nerve area") dies an abscess may form which creates pain on biting and touch, tenderness to palpating tissue near the involved tooth and possibly swelling. This is due to the infection at the tip of the tooth.
Also, there are situations when a tooth has an extensive large filling and requires a crown. In this case it may be indicated to electively have the endo treatment performed prior to the crown services.