Mouth odor is oral odor, the causes can be divided into oral causes, respiratory diseases, gastrointestinal diseases, metabolic abnormalities, etc., as follows. 1. Oral causes: food debris retained on the back of the tongue, shed cells, etc. is one of the main sources of oral odor, periodontitis, untreated caries in the oral cavity, poor restorations, food impaction, oral ulcers, etc. may also lead to oral odor. 2. Respiratory diseases: such as purulent tonsillitis, rhinitis, sinusitis, chronic pharyngitis. 3. Gastrointestinal system diseases: such as esophageal diverticulum, reflux esophagitis, etc. 4. Metabolic abnormalities: such as uncontrolled diabetes mellitus, renal failure and so on. There may be other reasons for the bad taste in the mouth, it is recommended to go to the hospital in a timely manner, to clarify the cause of the disease for targeted treatment.