Bad taste in the mouth may be the result of eating food with bad odor, such as onion, garlic, etc., or bad habits such as incomplete cleaning, pay attention to oral hygiene, regular dental cleaning will generally ease. Of course, it may also be caused by disease factors, including oral diseases, digestive diseases, respiratory diseases, nasal diseases and certain systemic diseases, etc. 1, oral diseases: periodontitis, gingivitis or oral ulcers, due to local inflammatory factors gathered, may emit odor or pus odor, treatment of oral diseases can control oral odor; 2, digestive diseases: indigestion, gastritis, gastric ulcers, gastroesophageal reflux disease or Gastroesophageal reflux disease or Helicobacter pylori infection and other conditions, may cause oral odor. Usually can be confirmed by gastroscopy, and breath test; 3, respiratory diseases: such as purulent bronchitis or lung abscess, etc., the gas expelled through the mouth can also be manifested as odor, accompanied by other infectious symptoms such as fever; 4, nasal diseases: such as purulent maxillary sinusitis or atrophic rhinitis can also lead to oral odor, usually accompanied by other symptoms of the nose; 5, systemic Diseases: Certain systemic diseases can also cause oral odor, such as diabetes mellitus ketoacidosis, can exhale rotten apple gas, other common are AIDS or chronic liver and kidney disease, etc.; 6, other: for example, lead, mercury poisoning and organic phosphorus poisoning, the mouth will also have an abnormal smell, usually with clear triggering factors. Therefore, if the patient’s mouth has been smelling bad for a longer period of time and there is no sign of relief or even aggravation, it is recommended to be alert and promptly identify the cause and then treat it.