Dry mouth (xerostomia) can result from a myriad of sources including age, polypharmacy, levels of daily water intake, or even presence of conditions such as diabetes.  Xerostomia is a concern to dentists because your teeth are constantly remodeling and laying down protective tissues to shield the vital parts of itself from infection.  The minerals that are needed for this process are dispensed to the teeth through the saliva.  If the flow of saliva is low or non-existent, the teeth will have less mineral to remodel with, which in time will leave them more susceptible to decay.  This form of decay will be slow, but constant.  That is why dentists sometimes prescribe a moisturizing mouthwash or highly fluoridated toothpaste to help keep the oral environment moist, in hopes that the teeth can keep remodeling in a more natural manner.

For more information follow the links below

https://www.mouthhealthy.org/en/az-topics/d/dry-mouth