Fluoride, usually in the form of sodium fluoride (NaF) in toothpaste, strengthens the enamel in your teeth. But how does it do that? Let’s back up a bit first.

Enamel is the outer layer of your teeth, and it’s made of a mineral called hydroxyapatite. It’s a calcium phosphate structure with one hydroxyl group (Ca₅(PO₄)₃(OH)). This mineral dissolves in the presence of acids, which is exactly what bacteria generate when they metabolize sugars in your mouth. This is how cavities form when you drink soda.

Fluoride ions help rebuild your tooth’s enamel by replacing the hydroxyl group in the apatite mineral. The new mineral ((Ca₅(PO₄)₃F), or fluorapatite, is more stable in the presence of acids, so your teeth are more resistant to decay.