When the second molar is rotten, it depends on how far the rottenness is. If the decay is only simple and does not damage the nerve of the tooth, it can be repaired directly. If the decay has passed the midline of the tooth, porcelain inlay can be made and bonded to the tooth. If the nerve is already exposed, you can kill the nerve to treat it. If the pulp is necrotic, and the pulp cavity is exposed, you can do the restoration of porcelain crown or porcelain inlay after the root canal treatment. If the tooth is so rotten that it passes through the bottom of the pulp chamber, the tooth needs to be extracted. If the tooth is so rotten that the gum is red and swollen, it needs oral anti-inflammatory medicine to reduce the swelling, you can take oral amoxicillin plus metronidazole tablets, roxithromycin, and it is usually recommended to extract the tooth in time after the swelling is reduced. If the rotten periapical abscess appears, it can be cut and drained when there is a sense of fluctuation, and it needs to open the pulp and take the nerve, and after taking the nerve, it also needs to do the restoration of porcelain crown. In addition, if the second molar is rotten, if the third molar is also found to be decayed, it needs to be extracted or repaired. Usually, if the second molar is decayed, it still needs to be preserved because it has a chewing function.