Valaciclovir and famciclovir, two new prodrugs (for aciclovir and penciclovir, respectively) have similar pharmacokinetics in many regards. Both have good but incomplete bioavailability, with the conversion to the active forms taking place in the liver, but by different cytosolic enzymes. Absorption and conversion are consistent in relevant patient groups, including those with liver disease. The pharmacokinetics of both active molecules are also similar in being mainly renally eliminated, a significant component of which is tubular secretion, and elimination half-lives from plasma of approximately 2.2 to 2.5 hours. Dosage adjustment is required in the presence of renal impairment. No clinically important drug interactions have been identified with either drug. The choice between the two agents is likely to depend on clinical factors such as tolerability, safety, efficacy, compliance and possibly cost, rather than on their pharmacokinetics.