Bad breath can be caused by lack of attention to oral hygiene, periodontitis, or diseases such as indigestion. If symptoms such as stomach pain or bloating occur, consider bad breath caused by indigestion and other stomach disorders. Put a clean gauze in your mouth and stir it with your tongue for a few seconds, then take it out. If there is an odor on the gauze, such as sour odor or putrid odor, consider that the bad breath is caused by not paying attention to personal hygiene or oral diseases such as periodontitis. If the bad breath is caused by gastric diseases such as indigestion, there may be a rotten apple odor or fecal odor after putting a clean gauze in the mouth and taking it out. The main reason is that after eating, the food cannot be digested in time and will react with harmful bacteria in a fermentation reaction, which will form a large amount of gas that will reflux into the esophagus, mouth and other parts of the body. Patients may also experience symptoms such as stomach bloating and a burning sensation in the epigastrium. If you want to know what causes bad breath, you can make an initial judgment based on your personal situation, and then seek medical treatment for relevant examinations to clarify the cause.