People are dry mouth when the glands in the mouth of saliva are not working properly. For this reason, might not be enough saliva to keep his mouth moist. There are several reasons why the salivary glands might not be working well.
Diseases
Sjogren’s Syndrome is a major cause of dry mouth.Other disorders can also cause dry mouth or affect the salivary glands. Some people feel the dry mouth, even if their salvary glands working properly. Some with certain diseases such as Parkinson’s disease or those who have suffered a stroke, may not be able to feel the humidity in the mouth and may think that your mouth is dry, although it is not.
Side effects of some drugs
More than 400 medicines can cause salivary glands to make less saliva. However, do not stop taking them without asking your doctor, your dose may have already been adjusted to help protect against the side effects of drying or chosen a drug less likely to cause dryness. These medicines include:

Antihistamines
Decongestants
Diuretics
Some drugs antidiarrhea
Some antipsychotic drugs
Tranquilizers
Some medicines for blood pressure
Antidepressants

Radiation therapy
The salivary glands can be damaged if exposed to radiation during cancer treatment.

Chemotherapy
Drugs used to treat cancer can make thick saliva, causing dry mouth.

Damage to the nerves
Injuries to the head or neck can damage the nerves that tell the salivary glands to make saliva.