How does fluoride prevent tooth decay?

Fluoride protects teeth in two ways:

Systemic fluoride. This is the fluoride in drinking water, fluoride vitamins or supplements. This fluoride is swallowed. Systemic fluoride in saliva strengthens the tooth structure and helps prevent decay. It reaches teeth still forming, even before you can see them in the mouth.

Topical fluoride. This is the fluoride in toothpaste, gels and rinses. It strengthens teeth already in the mouth. When drinking water is fluoridated, there is also a slight topical effect from the water. Both topical and systemic fluoride help prevent decay by strengthening teeth. Fluoride can help reverse early decay.