When a tooth that has had a root canal treatment hurts again, it is usually due to bacterial infection, underfilling of the root canal, and failure to heal. 1. Bacterial infection: When root canal treatment is performed, if the bacteria in the root canal are not removed, they will be retained in the root canal. Moreover, the use of various kinds of cold dental adhesive to fill the root canal will also produce a large number of cavities and the apical foramen is not completely closed, and it cannot be completely closed, so bacterial infection will be produced, causing the symptoms of pain. 2. Insufficient filling of root canal: After removing the tissue in the root canal, it is necessary to fill it. If the filling material chosen is not close, such as using temporary material or glass ionomer, it will easily lead to the entry of oral bacteria, thus causing pain. 3. Uncured: If the tissues in the root canal are not completely removed during root canal treatment, a small amount of viable pulp still remains in the root canal and inflammation may occur, so it will lead to pain in the patient’s tooth after root canal treatment. The reappearance of pain in the tooth after root canal treatment may also be due to conditions such as apical periapical inflammation, periodontitis, etc. It may also be due to trauma or fracture of the tooth, etc. It is recommended that the patient identifies the cause of the problem and undergoes targeted treatment.