I have been having persistent pain under my back tooth not long after having had a cavity repaired. That occurred more than 4 months ago. Since then the tooth has been sensitive to the point that biting down is painful. About a month ago it really hurt and became very sensitive to hot and cold. I called my dentist and he prescribed an anti-biotic over the phone. It worked and the pain went away after a 10 day dose. Now it is back again, having appeared two days ago. What to do? I will appear in person to the office tomorrow morning on an emergency basis. What can I expect and what should I demand to deal with this issue?

He will take x rays to determine infection at the base of the tooth probably necessitating removal of the filling, draining (dont let him send you home with the tooth packed for drainage). If the infection can be dealt with easily and it is isn't too advanced, then you will probably have to have a root canal.

Tickle is right. It sounds like you need a root canal. He might give you another round of antibiotics if he doesn't have time today to do the root canal but you should get in to have it done ASAP.