Fluoride—most frequently in the form of sodium fluoride, sodium monofluorophosphate, or stannous fluoride—combats tooth decay by strengthening enamel and, according to some researchers, by inhibiting bacteria's formation of acids that attack enamel. Although enamel is the hardest substance in the body, constant attack by the acids in saliva and plaque can dissolve it.
Fluoride's action is not completely understood, but chemists believe that the ingredient works through a series of chemical reactions called remineralization, in which fluoride is transformed into fluoride rich compounds called apatites that chemically bond with enamel to strengthen it and help protect it from dissolving in acid.
Calcium can help strengthen enamel, but excess calcium can contribute to tartar build-up. Calcium that is naturally present in saliva can be a source of excess calcium. Tartar control agents in toothpaste work by slightly changing the chemistry of saliva and teeth to inhibit the build-up of calcium on teeth. These changes inhibit calcium from crystallizing into tartar once the calcium binds to plaque. In this way, less tartar forms, and the tartar that does form is often easier to remove. Tartar control agents in toothpaste do not work below the gum line and thus have no clear effect on gingivitis.