Fluoride is not a medication, but rather a naturally occurring mineral most commonly found in air, soil, rock, plants and water; it is considered the most important mineral in the prevention and treatment of tooth decay.
Tooth decay occurs when acid produced by harmful bacteria attacks the surface of the tooth.
A constant low-level supply of fluoride is best to inhibit this. Fluoride in your drinking water and toothpaste acts like a constant 'repair kit' for your teeth.