Untreated necrosis of the tooth nerve may lead to periapical inflammation. Because the necrotic pulp tissue is more susceptible to bacterial infection, the bacterial infection progresses to the root tip, which can lead to periapical inflammation. Periapical infection, on the other hand, if acute, may cause severe pain and may also spread to the surrounding area, leading to facial interstitial infection and facial swelling and pain. There is also the possibility that periapical inflammation, because it is left untreated for a long time, may continue to develop to form a cyst at the tip of the root, which can cause resorption of the jawbone. If the cyst continues to grow, it may also lead to loosening of the tooth or invade the adjacent teeth, causing more damage. Therefore, if nerve necrosis of the tooth occurs, some patients may have no conscious symptoms but may experience discoloration of the tooth, which also needs to be treated as early as possible to prevent the development of the next disease.