The bad smell in the mouth is called oral odor, and its sources can be divided into bad breath originating from the oral cavity, such as tongue, periodontitis, and bad breath from sources other than the oral cavity, such as respiratory diseases, gastrointestinal system diseases, as follows. 1. Bad breath from the oral cavity: food debris and shed cells on the back of the tongue are degraded by bacteria and become the main source of oral odor. In addition, periodontitis can also lead to bad breath, the odor mainly comes from methyl mercaptan. 2. Bad breath from sources other than the oral cavity: chronic and purulent tonsillitis, rhinitis, sinusitis, chronic pharyngitis and other respiratory diseases, as well as esophageal diverticulum, reflux esophagitis and other gastrointestinal disorders, may lead to oral odor. There may be other causes of bad breath, it is recommended to go to the hospital in a timely manner, to clarify the cause of the disease and then targeted treatment.