Oral acyclovir therapy for herpes zoster has been studied in double-blind, placebo-controlled trials of two dosages, 400 mg and 800 mg five times per day for 10 days. Compared with placebo recipients, recipients of the high-dosage acyclovir experienced a significantly shortened period of viral shedding, significantly accelerated time to 50 percent scabbing, significantly accelerated time to 50 percent healing, and after two days of therapy, significantly less frequent formation of new lesions. The duration and severity of acute pain were less in acyclovir recipients, with differences in pain severity achieving statistical significance (p = 0.03) between Days 3 and 10 and correlating with the treatment differences in new lesion formation. In studies of the 400 mg five times per day dose schedule, differences between acyclovir and placebo recipients were not significant. In a six-month follow-up of recipients in the higher dosage study, the acyclovir recipients experienced less post-zoster pain than placebo recipients; differences in the prevalence of pain were most significant for the presence of a persistent pain in the first three months of follow-up. Oral acyclovir at these dosages appears to be free of adverse reactions. In summary, oral acyclovir at a dosage of 800 mg five times per day for 10 days for treatment of acute herpes zoster is superior to 400 mg five times per day and favorably alters the course of the disease.