Overall, acyclovir is a remarkably safe drug considering its potent antiviral effect. The most frequent reactions with short-term use of oral acyclovir are nausea and vomiting and with 6 months' use headache, diarrhea, nausea, and vomiting. These symptoms are also seen frequently with placebos. The most frequent adverse reaction to intravenous use has been inflammation and phlebitis at the injection site. The two most important serious adverse effects are (1) encephalopathic changes with abnormal electroencephalograms and lethargy, tremors, confusion, and seizures and (2) renal precipitation of the drug because of a rapid bolus of drug administered parenterally. Safety of acyclovir for use during pregnancy and in neonates and young children has not been established.