The common causes of bad taste in the mouth include digestive system diseases, dental related diseases, kidney related diseases, long-term smoking, food residue in the mouth and other causes. 1. Digestive system diseases: some patients have a bad taste in the mouth because of the weakening of gastrointestinal function, which leads to the retention of food in the stomach for too long. You should eat a light diet, and if necessary, you can take gastrointestinal power drugs (mosapride) to promote gastric emptying of food. 2. Dental diseases: periodontitis or dental caries and other oral diseases, should be cleaned in time after eating, to prevent the occurrence of appealing diseases. 3. Kidney-related diseases: some patients because of kidney dysfunction, resulting in metabolites can not be discharged in a timely manner, the accumulation of the body, there may be a taste of ammonia in the mouth, commonly found in the late stage of uremia patients. Pay attention to rest, low-potassium diet, and regular testing indicators. 4. Long-term smoking: some long-term smoking patients may also have a bad taste in the mouth, it is recommended to quit smoking in time and pay attention to clean the mouth after meals. 5. Food residue in the mouth: not cleaning the mouth in time after eating, resulting in food being reproduced by bacteria in the mouth, thus resulting in a bad taste in the mouth. It is recommended to clean the mouth in time after eating. If the patient has a bad taste in the mouth for a long time, it is recommended to go to the hospital in time to clarify the cause. Especially by diabetic patients and kidney disease patients.