There are numerous anti plaque medications which reduce the number of bacteria in the oral cavity hence preventing tooth decay.
I will list a few that are used:
The most important agents which prevent tooth decay are Chlorhexidine and Triclosan.
This is used mainly as a mouthwash or gel. It reduces the most important cariogenic bacteria, Streptococcus mutans. This should not be used continuously as it may cause staining of teeth.
It is included as an active agent in toothpaste and gels. This is non-irritant, tasteless, non-staining and bacteriostatic.
Tooth decay can only be prevented by these agents and not cured once it commences. So proper oral care accompanied by preventive measures is vital for proper oral hygiene.