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No clinically significant interactions have been identified.
Acyclovir is excreted primarily unchanged in the urine via active renal tubular secretion. Any medicinal products administered concurrently that are excreted the same way may increase acyclovir blood concentrations following Valacyclovir administration.
Following administration of 1 g of Valacyclovir, cimetidine and probenecid increase the AUC of acyclovir by this mechanism, and reduce acyclovir urine excretion. However, no dosage adjustment is necessary at this dose because of the high therapeutic index of acyclovir.
In patients receiving higher doses of Valacyclovir (8 g/day) as cytomegalovirus prophylaxis, caution is required during concurrent administration with medicinal products which compete with acyclovir for elimination, because of the potential for increased blood levels of one or both products. Increases in AUCs of acyclovir and of the inactive metabolite of mycophenolate mofetil, an immunosuppressant agent used in transplant patients, have been seen when the medicinal products are coadministered.
Caution is also required (with monitoring for changes in renal function) if administering high doses of Valacyclovir with medicinal products which affect other aspects of renal function (e.g. ciclosporin, tacrolimus).
