To understand modern fluoride used in the prevention of decay, it is necessary to investigate how decay begins.
Hydroxyapatite crystals make the external surface of natural healthy tooth enamel. Because we humans have an affinity to eating sugar, and our bodies carry bacteria (S. Mutans) that also loves sugar and secretes acidic waste, we have a necessary battle to maintain the strength of our teeth. There continues a dynamic chemical exchange with the oral environment. A hydroxyl group (OH-) has a relatively weak bond forming the tooth surface as hydroxyapatite crystals. By using fluoride (Fl-), tooth paste and rinses, the exchange of the bonded molecule happens: Instead of (OH-), relatively weak bond, fluoride (FI-) is inserted at that same place. Because the (FI-) bond is a stronger bond, the bacterial toxin has a harder time breaking and “invading” the tooth.
Both fluoride toothpaste and rinses have the same mode of action on the external tooth surface. Cleaning with the toothbrush bristles and floss, the outer surface allows the fluoride to wash over and make this exchange, which resists cavities (tooth decay).
Historically, some communities natural water supplies were found to contain higher fluoride content. These same communities where experiencing lower tooth decay index. Correlation studies began the basis of fluoride use in decay prevention. It is effective to have a small amount of fluoride in drinking water, but this will only affect tooth strength during the pre-born development up to age 6. These are the only years, when the tooth is forming, that would benefit with internal fluoride. (The roots of the teeth form later than age 6 but are not at risk for decay because they are below the gum line).
Fluoride toothpaste and rinses are designed to work topically, not internally. The most important step of recognizing fluoride effect is cleaning the tooth carefully before, so that the fluoride is able to make direct contact. This is why after a dental office cleaning, fluoride rinse or polishing varnish with fluoride is most effective.
God Bless and be healthy.