How Fluoride Works to Prevent Tooth Decay

Fluoride is the fluorine ion added to toothpaste and dental rinses to help protect your teeth from cavities. While increasing the concentration of systemic fluoride (e.g., through fluoridating drinking water) has not been proven to reduce the incidence of tooth decay, direct contact between fluoride and teeth strengthens and helps remineralize damaged enamel.

The calcium and phosphate compound which makes up tooth enamel is a modified form of hydroxyapatite, which is susceptible to attack by acids. Bacteria that thrive on the sugars found in the mouth multiply on dental surfaces and produce acids.

Mechanical brushing of teeth dislodges these bacteria and rinses them away, but doesn't do anything to address the ongoing damage to enamel from acid exposure. Fortunately, saliva neutralizes these acids and teeth use compounds found in saliva to remineralize the enamel. So, as long as the rate of demineralization and the rate of remineralization remain in balance, teeth remain strong and healthy.

When more minerals are lost from teeth than are replaced, dental caries or cavities form. Fluoride helps by interacting with hydroxyapatite to form a stronger compound that is less susceptible to acid attack. Also, fluoride remineralizes damaged enamel (though it can't fix a cavity once one has formed). There is some evidence fluoride may inhibit the growth of bacteria on teeth or limit their capacity for producing acid.

Even though you spit out toothpaste and probably rinse your mouth, enough fluoride remains on your teeth and in your saliva to confer protection for a while after brushing your teeth or using a fluoridated rinse.