Early treatment, with antiviral therapy within the first 72 hours of blistering, is important to promote timely healing of the lesions and to reduce the pain caused by herpes zoster. Acyclovir, famciclovir and famciclovir are guanosine analogues that inhibit viral DNA polymerase synthesis after phosphorylation by thymidine phosphatase to the triphosphate form. The number and dosage of daily doses are determined by oral bioavailability. However, patient compliance decreases as the number of daily doses increases. Common side effects include nausea, headache, and gastrointestinal reactions, but these drugs are otherwise safe and well tolerated. Patients with renal insufficiency require dose adjustment because these drugs are excreted through the kidneys.