I have a molar (upper right, second from the back) that had a root canal in the 80s.That root canal failed in April 2012 and was addressed with a root canal retreatment from a referred endo. One week after, it abscessed and the endo phoned in antibiotics for me. 2 weeks ago, xrays show this tooth with a periapical abscess and sudden, constant throbbing pain. Again referred to an endo who wants to do a scan before final decision but is currently leaning toward re-treating the tooth.