It may be because of yin deficiency and excessive fire, generally because of staying up late, overeating and spicy stimulation, or excessive alcohol and tobacco, resulting in insufficient vaginal fluid, excessive internal heat, dry mouth and bitter taste. It is suggested to pay attention to the regular diet and eat less spicy food. You can cook Ophiopogon japonicus, Asparagus and Lycium barbarum soup to nourish yin and improve dry mouth.