While active against Herpesviridae, oral aciclovir is limited by low and inconsistent bioavailability. Modification of aciclovir by valine esterification, producing valaciclovir, results in significant increases in systemic aciclovir plasma levels. The exact mechanism of increased absorption with valaciclovir is not fully determined but probably involves intestinal dipeptide transporters, followed by rapid ester hydrolysis in the small intestine and liver. The enhanced pharmacokinetics of valaciclovir have translated into improvements in clinical efficacy and patient convenience.