A dry mouth means that there is not enough saliva to maintain adequate moisture mouth. Everyone has dry mouth at one time, especially if you are nervous, upset or stressed. But if you continually, or almost dry mouth, it can be uncomfortable and cause more serious diseases or may be a sign of a serious medical problem. Saliva in addition to its role as a humidifier of the mouth, helps to digest food, protects teeth from decay, prevents infection by fighting against bacteria and facilitates chewing and swallowing.
There are several reasons malfunctioning salivary glands:
Side effects of certain medications - More than 400 medications can cause dry mouth including antihistamines, decongestants, painkillers, diuretics and medicines for hypertension and depression. 
Disease - disease with action on the salivary glands, such as diabetes, Hodgkin's disease and Parkinson's, HIV / AIDS and Jorgen’s syndrome, can cause dry mouth.
The radiation-salivary glands may be damaged if the head or neck is irradiated during treatment for cancer. The lack of saliva can be total or partial, permanent or temporary.
-The chemotherapy drugs used to treat cancer can make saliva thicker or filamentous and give an impression of dry mouth.
Menopause - The rate change hormone acts on the salivary glands and often leaves an impression of persistent dry mouth during menopause or post-menopause.
Smoking - Many of pipes smoking, cigars and cigarettes suffer from dry mouth.