Acyclovir is an antiviral agent. It is characterized by direct action against the virus. It is used for therapeutic and prophylactic purposes for various types of herpes infection. It has pronounced, selective activity against viruses: Herpes simplex types 1 and 2, Varicella zoster, Epstein-Barr, cytomegalovirus. The action is aimed at both treatment (the likelihood of new elements, visceral complications is reduced, crusts form faster, pain in shingles is reduced) and prevention of relapses. Acyclovir is a structural analogue of purine nucleosides (normal DNA components). Selective antiherpetic activity is due to the affinity for thymidine kinase of certain viruses, under the influence of which and other enzymes it is activated (converts to acyclovir triphosphate) and prevents the reproduction of viruses.