Disclosed is a H-phosphonate nucleoside of the formula (I) or a pharmaceutically acceptable salt, a stereoisomeric, tautomeric or polymorphic form thereof, wherein RY is alkyl, alkenyl, alkynyl, aryl, aryl alkyl, cycloalkyl, cycloalkenyl, hydroxyalkyl, amino, heterocyclyl or heteroaryl R1 is a moiety derivable by removal of a hydrogen from a hydroxy group of an anti-viral drug for the treatment of a host infected with a Flaviviridae virus or hepatitis B virus.
