Acyclovir (ACV) has a novel, highly selective biological activity which results in the inhibition of herpesvirus replication at concentrations 300-3000-fold lower than those that will inhibit mammalian cellular functions. Subacute and chronic studies in animals indicate that the drug is relatively non-toxic, poses no special risks to pregnancy or the foetus and does not induce detectable oncogenic or genetic changes. In man, acyclovir has a plasma half-life of approximately three hours, is widely distributed throughout the body tissues and is rapidly cleared, mainly as unchanged drug, through the kidneys. Provided that the product (Zovirax) is used according to the manufacturers' instructions, the risk of significant toxic effects is low.