Pain after root canal treatment for endodontitis may be caused by medication or instruments, or by inflammation that has not been eliminated. 1. Pain caused by drugs or instruments: Root canal treatment after endodontitis requires the use of instruments to go deep into the root canal to remove the bacteria on the inner wall of the root canal, and at the same time the use of medication rinsing, during the operation process, the instruments or medication may stimulate the periapical tissues, resulting in temporary pain, usually the pain will be gradually relieved, and can be observed and treated first. 2. Inflammation has not been eliminated: it is possible that endodontitis gradually develops into periapical periodontitis, and the inflammation of periapical tissues causes pain after root canal treatment, if the pain worsens, it is necessary to continue to change the medicine or consider apical surgery. Endodontitis root canal treatment pain will generally be gradually relieved, if the pain aggravation is recommended to see a doctor as soon as possible.