A root canal has a worse reputation than it deserves. In practice it is the treatment that stops the pain rather than causes it, and it lets you keep a tooth that would otherwise need extracting.
When decay or a crack reaches the nerve inside a tooth, the pulp becomes infected. That is the throbbing, keeps-you-awake kind of pain, and it does not settle on its own. Treatment means clearing the infection out of the canals, disinfecting them, and sealing the space so bacteria cannot get back in.
It is done under local anaesthesia, at a pace you are comfortable with. Depending on the tooth, it takes one or two visits.
Afterwards the tooth is more brittle than it was, because the pulp that kept it hydrated is gone. That is why a root canal is finished with a crown rather than just a filling, particularly on back teeth that take real biting force.
