Brivudine is an analog of thymidine, and is incorporated into the viral DNA. It blocks the action of DNA polymerases, thus inhibiting viral replication. It has a stronger antiviral effect against the varicella-zoster virus compared with reference compounds such as aciclovir or penciclovir. It has high, selective activity against varicella zoster virus (VZV), inhibiting VZV replication, possibly through competitive inhibition of viral DNA polymerase, or by acting as an alternative substrate to deoxythymidine triphosphate, causing viral DNA strand breakage.