Bitter mouth may not be a sign of serious illness, but may be caused by improper diet or gastroesophageal reflux disease (GERD). 1. Improper diet: If you eat foods such as bitter melon and dark chocolate, you may experience bitter mouth, which is normal and not a cause for excessive concern. 2. Gastroesophageal Reflux Disease (GERD): GERD may be caused by the decrease of anti-reflux function, the decrease of esophageal clearing ability, and the weakening of esophageal mucosal defense barrier. Patients may experience reflux, which refers to the influx of stomach contents into the oral cavity, and some patients may feel bitter in the mouth. There are many causes of bitter mouth, it is recommended that patients exclude physiological factors, and then go to the regular hospital for relevant examinations, so as to clarify the cause of the disease, in order to avoid delaying the condition.