A variety of pyrimidine nucleosides (both halogenated and nonhalogenated) effectively inhibit the replication of herpes simplex viruses with limited toxicity to host cells. The mechanism of action of these compounds appears to be substitution of pyrimidine for thymidine, causing defective DNA molecules.
Idoxuridine is effective as a topical treatment for herpesvirus infection of the superficial layers of the cornea (herpesvirus keratitis) and of the skin; when administered systemically, however, it is toxic.
Trifluridine, is currently the agent of choice to treat herpesvirus keratitis in humans. No other antiviral pyrimidine nucleosides have been used clinically to any notable extent.