The causes of bad breath are as follows: 1, the oral cavity internal causes: problems in the oral cavity have gum problems, such as gum inflammation, redness, swelling, bleeding, etc., will cause the bacteria in the oral cavity to produce rotting odor leading to bad breath; teeth itself, such as teeth above the caries, caries residual food residue in the inside of the caries, will be fermented to cause odor; and then there is a food implantation, the residual food in the gap between the teeth, if the food is long enough, it will produce Corruption of the breath, which is the cause of bad breath in the oral cavity; 2, other problems related to the oral cavity: the oral cavity as the digestive tract, respiratory tract of the common opening, digestive problems will also affect the oral odor, such as gastrointestinal reflux, acid reflux, etc., the smell will be reflected through the oral cavity. Then there are respiratory problems that can also be reflected through the mouth, such as tonsillitis or lung problems as well. Then there are metabolic disorders that may also have an effect on oral odor. In short, the causes of bad breath are complex, there are oral causes, there are respiratory and digestive tracts connected to the oral cavity, so it is necessary to identify the cause of the disease in order to target treatment.