[A synthetic thymidine analogue with potent antiviral activity against herpes simplex and varicella zoster viruses. Sorivudine is phosphorylated into triphosphate form within the cells and this metabolite interferes with viral DNA replication by inhibiting DNA polymerase activity without been incorporated into elongating viral DNA. Due to the dangerous effects of drug-drug interactions via its metabolite, sorivudine has been removed from the market. ( NCI )]