Light micrograph (LM) of Acyclovir crystals. Acyclovir (aciclovir), marketed as Zovirax, is one of the main antiviral drugs used in the treatment of herpes infections. Its discovery has been seen as the start of a new era in antiviral therapy, as it is extremely selective and low in cytotoxicity. However, it has a very narrow spectrum, only effective against certain viruses such as HSV-1, HSV-2, VZV and EBV, but has hardly any effect against human cytomegalovirus (CMV). It does not eradicate latent herpes, and does not work very well against genital herpes in women. Acyclovir is rather different from other nucleoside analogues, for it contains only a partial nucleoside structure as the sugar ring is replaced by an open-chain structure. Magnification: x40 when shortest axis printed at 25 millimetres.