Intravenous acyclovir is effective for varicella in adults and immunocompromised children, causing more rapid resolution of the illness and fewer complications. Intravenous acyclovir in immunocompromised patients with herpes zoster decreases new lesion formation, decreases acute pain, halts dissemination of the virus, and lessens visceral complications. Intravenous acyclovir may also be effective in zoster encephalitis. Intravenous vidarabine also has a favorable affect on chickenpox and herpes zoster. Topical acyclovir may benefit herpes zoster in immunosuppressed patients by accelerating cutaneous healing. Oral acyclovir appears to be effective in varicella and zoster in immunocompromised patients. It is also effective in otherwise normal patients, but its effect seems less dramatic and the drug must be given early. Neither acyclovir nor vidarabine has been proven clearly to prevent postherpetic neuralgia. Because varicella zoster virus is less sensitive to acyclovir than is herpes simplex, intravenous doses of 500 mg/m2 or 10 mg/kg every 8 hours or oral doses of 800 mg five times a day are recommended. At these doses adequate hydration and urine flow must be maintained, the mental status of the patient must be monitored, and impaired renal function requires regulation of dosage downward.