Valaciclovir may decrease the excretion rate of Tenofovir disoproxil which could result in a higher serum level. The therapeutic efficacy of Varicella Zoster Vaccine (Live/Attenuated) can be decreased when used in combination with Valaciclovir. Valaciclovir may increase the nephrotoxic activities of Foscarnet. Valaciclovir may increase the central nervous system depressant (CNS depressant) activities of Zidovudine. The serum concentration of Tizanidine can be increased when it is combined with Valaciclovir. The serum concentration of Mycophenolic acid can be increased when it is combined with Valaciclovir.