As the name implies, dry mouth occurs when the salivary glands stop producing a normal amount of saliva. Normally, saliva helps to prevent cavities in two big ways. First, it helps to dissolve the foods we eat and prevents stickier food particles, like starches, from clinging to tooth enamel. Second, saliva helps to restore the structural strength of tooth enamel by delivering a continual source of minerals to each tooth in the mouth.
When saliva production is reduced, it becomes easier for food particles to cling to the surfaces of the teeth. Over time, this leads to the cultivation of oral bacteria and tooth decay. Less saliva also means that fewer minerals are delivered to the teeth, making it easier for cavities and dental caries to form in the enamel of each tooth.