Question: What is (are) Dry Mouth ?
Answer: Saliva does more than keep your mouth wet. It helps digest food, protects teeth from decay, helps to heal sores in your mouth, and prevents infection by controlling bacteria, viruses, and fungi in the mouth. Saliva is also what helps us chew and swallow. Each of these functions of saliva is hampered when a person has dry mouth.
Source: NIHSeniorHealth

Question: What causes Dry Mouth ?
Answer: Yes. More than 400 medicines, including some over-the-counter medications, can cause the salivary glands to make less saliva, or to change the composition of the saliva so that it can't perform the functions it should. As an example, medicines for urinary incontinence, allergies, high blood pressure, and depression often cause dry mouth.
Source: NIHSeniorHealth