Acyclovir is active only against herpes group of viruses; H. simplex type I is most sensitive followed by H. simplex type II > varicella-zoster virus= Epstein-Barr virus; while cytomegalovirus (CMV) is practically not affected. Both H. simplex and varicella-zoster virus have been found to develop resistance to acyclovir during therapy; the former primarily due to mutants deficient in thymidine kinase activity and the latter primarily by change in specificity of virus directed enzyme so that its affinity for acyclovir is decreased. Use Acyclovir is effective in patients with normal as well as deficient immune status. 1 . Genital Herpes simplex Generally caused by type II virus; can be treated by topical, oral or parenteral acyclovir depending on stage and severity of disease. 2. Mucocutaneous H. simplex is a type I virus disease, remains localized to lips and gums; does not usually require specific treatment, but acyclovir skin cream may provide some relief 3. H. simplex encephalitis (type I virus): Acyclovir 10 to 20 mg/kg/8 hr i.v. for ;?.10 days is the drug of choice. Treatment is effective only if staed early: delay precludes salutary effect on moality and neurological complications. 4. H. simplex (type I) keratitis: Acyclovir is equally effective as idoxuridine in superficial dendritic corneal ulcer, and may be better for deep stromal infections because of good corneal penetration. Though acyclovir eye ointment acts slower than idoxuridine eye drops, blindness can be prevented. The eye ointment should be applied 5 times daily till 3 days after healing. 5. Herpes zoster: The varicella-zoster virus is less susceptible to acyclovir. As such, higher doses are needed and it should be used only in immunodeficient individuals or in severe cases: 10 mg/ kg/8 hr i.v. for 7 days. 6. Chickenpox: in patients with immunodeficiency and in neonates only calls for specific therapy. Acyclovir (15 mg/kg/day i.v. x 7 days) is the drug of choice: reduces fever, eruptions, hastens healing and prevents visceral complications ESSENTIALS OF MEDICAL PHARMACOLOGY K.D.TRIPATHI SIXTH EDITION PAGE NO:768,769