Precursor of Penciclovir which is used for treatment of infections due to Herpes zoster and Herpes simplex viruses. At latest update, relevant published data on excretion into breast milk were not found. Possibly, a low risk medication during breastfeeding, since similar pharmacokinetics and side-effects are shared with well known drugs pertaining to the same family. Until more data on this medication is available, safer alternative drugs are preferred, especially in such critical age periods.