Precursor of Aciclovir which is used for treatment of infections due to Herpes zoster and Herpes simplex viruses and the prevention of Cytomegalovirus infection in immunocompromised patients. It is excreted into breast milk as Aciclovir in non-significant clinical amount, with no side-effects observed among breastfed infants from treated mothers. Urine levels of Aciclovir in those infants were shown to be very low.