The main causes of bad breath in the mouth are: 1, dental caries, due to the existence of dental caries in the mouth, there are a large number of bacteria and food residues in dental caries, a large number of food residues fermentation under the action of bacteria, resulting in bad breath. 2, infection foci in the mouth, such as purulent tonsillitis, oral ulcer infection and periodontitis, etc., due to the existence of bacterial infection in the mouth, a large number of bacteria breeding leads to bad breath in the mouth. 3, gastric Helicobacter pylori infection, due to the infection of Helicobacter pylori food in the stomach, under the action of Helicobacter pylori gas production and heavy smell, resulting in bad breath. 4, dietary factors, mainly food with a heavy smell, such as garlic, onions and other foods, after eating the mouth will appear odor and lead to bad breath. 5, bad habits, such as smoking, not Brush your teeth can lead to the growth of bacteria in the mouth and halitosis.