Synthetic drug and nucleoside analogue which is structurally related to guanine with antiviral activity. It is used in infants and newborns for treatment of respiratory syncytial virus (RSV) and adenovirus. Along with interferon for treatment of hepatitis C (authorized on these cases from the 3 years of life). At latest update no published data on excretion into breast milk were found. Its large volume of distribution makes it unlikely that may occur the passage of significant amounts to the milk. Its low oral bioavailability hinder the passage toward infant’s plasma from ingested milk, except in preterm milk and immediate neonatal period, for which there may be an increased intestinal permeability. Since it is used as treatment on infants and its pharmacokinetic data are favorable, it should be regarded as compatible with breastfeeding for short-term exposures, however, to be cautiously used on long-term treatments.