Bad breath is usually divided into two types: physiological halitosis and pathological halitosis. Bad breath is usually caused by poor oral hygiene, indigestion, oral ulcers, dental caries, gingivitis, rhinogenic diseases, respiratory diseases, etc. 1, poor oral hygiene: if you do not brush your teeth on time is likely to lead to fermentation of food left in the mouth to produce bad odor; and if saliva secretion is low, dry mouth, the ability to destroy bacteria is low. These bacteria ferment in the mouth to produce bad odor, which can lead to bad breath. 2, indigestion: usually if a long time diet, overeating will increase the body’s gastrointestinal burden, the spleen and stomach will therefore cause damage, a long time will lead to gastrointestinal diseases and digestive system disorders, etc., there will also be bad odor in the mouth. 3, mouth ulcers: mouth ulcer patients will not only occur bad breath, usually accompanied by bloody saliva. Ulcer trauma is very easy to breed the growth of a variety of corrosive bacteria, these corrosive bacteria in the process of reproduction, growth, will produce odorous substances, halitosis. 4, the occurrence of inflammation: common patients with dental caries, gingivitis, respiratory diseases, rhinogenic diseases will have bad breath, often due to prolonged fermentation of food or gastrointestinal disorders caused by. Patients with bad breath are recommended to go to the hospital for examination to identify the cause, and to strengthen the care of the mouth in daily life and keep the good habit of brushing teeth.