Doris has always enjoyed chocolate and drinks. Recently, she felt some soreness in her upper right back tooth while eating dessert, and she also had a food blockage, so she took time to visit a nearby hospital. After examination, the doctor said that the tooth was decayed and had reached the pulp, leading to pulpitis, which required root canal treatment. After several treatments, the tooth still had pain when eating, so Doris came to the Department of Stomatology at Sun Yat Sen Memorial Hospital of Sun Yat-sen University, where the doctor diagnosed pulpitis and redid the root canal treatment. Root canal treatment is now an internationally recognized effective method for treating endodontic and periapical diseases, and with the increasing improvement of root canal treatment technology, the probability of tooth preservation is greatly increased. However, due to various reasons, there are always some cases where the tooth is still painful after root canal treatment and normal function cannot be restored. The main reasons for the occurrence of pain after root canal treatment are as follows: 1. Root canal underfill is the most common reason for root canal treatment failure. When the root canal is underfilled, the root tip is poorly closed and tissue fluid can seep into the root canal, leading to bacterial colonization in the root canal and causing acute and chronic periapical inflammation. 2. Pain caused by periapical tissue damage is also a common cause. When the root canal is cleaned and enlarged, the chronic inflammation of the root tip is provoked and acute periapical inflammation occurs in the affected tooth. 3, the tooth pulp is not cleaned. The root canal is not completely cleaned during root canal treatment, and the pulp still remains alive in the root canal, which produces pain under the combined effect of mechanical injury, drug stimulation and inflammation. 4.The root canal is missed. The root canal system of teeth is very complex, and the root canals of some teeth often have variants. If a root canal is missed in the root canal treatment process and the perfect root canal treatment is not completed, it may become a lesion and lead to residual pulpitis or periapical inflammation. 5. Poor root canal disinfection. Bacterial infection is the main cause of pulpitis and periapical inflammation, and poor root canal disinfection can lead to root canal failure if more bacteria remain in the root canal. 6, drug pain. This is related to the nature of the drugs used in root canal treatment itself and the individual differences of patients. In order to remove the infected material from the root canal as much as possible, root canal flushing and root canal sealing disinfection are needed during root canal treatment, and individual patients may react significantly to the root canal disinfection drugs commonly used in clinical practice, resulting in pain. 7. Tooth splitting. Some patients choose to fill the tooth after root canal treatment without doing braces restoration in time, as a result, the tooth splits after biting hard objects and causes pain, but then the tooth may not be retained. Most of the teeth with pulpitis and periapical disease that need root canal treatment have been destroyed, and the amount of remaining tooth tissue is small, and because the tooth has lost the nutritional supply from the pulp, the brittleness of the tooth increases and the fracture resistance decreases, and the tooth is prone to splitting when it is subjected to chewing pressure. Therefore, timely placement of braces on the tooth after root canal treatment can greatly reduce the risk of tooth fracture. There are many reasons for the occurrence or aggravation of pain after root canal treatment, mainly related to the complexity of the root canal system of the tooth, the limitations of existing root canal instruments and methods, and the individual differences of patients. Pain can be avoided or reduced to some extent by careful examination and careful operation of the doctor during root canal treatment, and of course, the cooperation of the patient during the root canal treatment is also very important.