It is a fluorinated pyrimidine nucleoside analog. It is structurally very similar to thymidine, the only difference being the replacement of a methyl group on the pyrimidine ring of thymidine with a trifl uoromethyl group. Once converted to the triphosphate, the agent is believed to competitively inhibit the incorporation of thymidine triphosphate into viral DNA and, to a lesser extent, to be incorporated into viral DNA, leading to the synthesis of a defective DNA that renders the virus unable to reproduce. Trifl uridine monophosphate is an irreversible inhibitor of viral thymidine synthase. Trifluridine is active against HSV-1, HSV-2, and vaccinia virus. It is generally considered to be the drug of choice for treatment of HSV keratoconjunctivitis and recurrent epithelial keratitis. Because the triphosphate form of trifl uridine can also incorporate to some degree into cellular DNA, the drug is considered to be too toxic for systemic use. Therefore, the use of trifluridine is restricted to topical application as a solution to the eye. A short half-life of approximately 12 minutes necessitates that the drug be applied frequently. Side eff ects include a transient irritation of the eye and palpebral (eyelid) edema