There is a high success rate for root canals and they provide relief from pain for most people. In the United States, over 15 million root canals are performed every year by endodontists and dentists. Over 95% of those root canals are successful. In a successful root canal, the natural tooth is preserved and its life is extended to match that of the other healthy teeth. The root canal can, however, require retreatment if it is not successful.   Symptoms may return a few months to a few years after an unsuccessful root canal. Endodontic retreatment can sometimes provide relief and restore health and balance to the mouth. You may experience symptoms such as increased sensitivity to temperature, constant pain or pressure, swollen gums, discharge or pus, intense pain around the tooth, a consistent ache in a specific area, a bad taste in the mouth, and more.