Sourness in the tooth one year after root canal treatment may be caused by some residual pulp in the root, apical periodontitis, or dentin hypersensitivity in other teeth. In a tooth after root canal treatment, the tooth cannot feel hot, cold, or sweet because the nerve has been taken out. If the tooth is sore one year after root canal treatment it may be caused by a small amount of residual pulp, which is a living nerve, in the root canal or by periapical inflammation. It can be relieved by root canal retreatment of the tooth. It may also be caused by decay in other teeth. Wedge-shaped defects in the neck of the tooth caused by horizontal brushing can also cause tooth acidity. Therefore, tooth acidity one year after root canal treatment is mostly seen in residual pulpitis, apical periodontitis, or dentin sensitivity caused by other tooth caries and wedge-shaped defects. A detailed examination in the dentistry is needed.