When teeth are attacked by acid from our diet and acid produced by the bacteria in the mouth, they loose calcium and phosphate minerals. This is called demineralization. Once the acid level drops, the teeth can reabsorb some minerals; this is called remineralization. The presence of low levels of fluoride in the mouth encourages remineralization and improves the strength of the tooth enamel and its ability to resist acid attack. Another way in which fluoride helps protect teeth is by reducing the ability of the plaque bacteria to produce acid.
Just drinking public water will provide some protection. However, health professionals have endorsed the practice of supplementing our intake with certain dietary products, and topical fluorides in many toothpastes and some oral rinses. Certain beverages such as tea may also contain fluoride. Dental varnishes and gels may also be applied directly to teeth by a health professional to boost fluoride intake.
It is generally not safe to swallow large amounts of toothpastes, rinses, or other products containing topical fluoride. In rare cases, some people may be overexposed to high concentrations of fluoride, resulting in a relatively harmless condition called fluorosis, which leaves dark enamel stains on teeth.