There are three most common types of fluoride. Acidulated phosphate fluoride (APF), neutral sodium fluoride (NaF), and stannous fluoride. APF is great against enamel caries, promotes uptake to harden areas that had begun to demineralize. NaF is good for hardening enamel and can help reduce sensitivity when applied regularly. Stannous fluoride covers a broader range. It is effective against enamel caries, reduces sensitivity and has bactericidal properties, especially against bacteria that cause root decay and even reduces gingivitis.