Idoxuridine is an anti-herpesvirus antiviral drug. It is a nucleoside analogue, a modified form of deoxyuridine, similar enough to be incorporated into viral DNA replication, but the iodine atom added to the uracil component blocks base pairing. It is used only topically due to cardiotoxicity. It was synthesized by William Prusoff in the late 1950s. Initially developed as an anticancer drug, idoxuridine became the first antiviral agent in 1962.