Valaciclovir Cimetidine

De Bony F, Tod M, Bidault R, et al. Multiple interactions of cimetidine and probenecid with valaciclovir and its metabolite acyclovir. Antimicrob Agents Che-mother 2002 46 458 163. 

ACICLOVIR/ VALACICLOVIR H2 RECEPTOR BLOCKERS -CIMETIDINE t efficacy and adverse effects of antivirals Competition for renal excretion Use doses >4 g/day valacidovir with caution or consider alternative acid suppression. For doses <1 g/day, interaction is not thought to be clinically significant. Studies available only for valacidovir... 

Twelve healthy subjects were given a 1-g dose of valaciclovir alone or with cimetidine 800 mg, taken 10 hours and 1 hour before. The AUC0.3 for the prodrug valaciclovir was increased by 73% by cimetidine, and the AUCo.24 for the active metabolite of valaciclovir, aciclovir, was increased by 27%. The renal clearance of aciclovir was reduced by 22%, although the total urinary recovery of aciclovir was unchanged. ... 

These interaetions are established but, because aciclovir has such a wide therapeutie index, the authors of the study suggest that its interaetion with eimetidine is probably elinieally unimportant. It seems likely that no ehanges in the usual dosages of aciclovir or valaciclovir will be needed in patients also taking eimetidine. However, the UK manufaeturer states that eaution is required with high doses of valaciclovir, and that alternatives to eimetidine could be considered in this situation. No special precautions would seem necessary if cimetidine is used with famciclovir. 

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