Root canals are typically needed when there is an infection present deep within a tooth. Upon your examination appointment the dentist will take X-rays to determine where and how deep the decay is. An antibiotic may need to be prescribed to calm the infection before scheduling the treatment.
Unlike Hollywood's rendition, it is a myth that root canal therapy hurts. The feeling is no different than getting a filling done.
Diseased material is removed and the area is disinfected and sealed. The intent of this procedure is to save the existing tooth and prevent the potential of the disease spreading and affecting other teeth.