Valacyclovir Cimetidine
ModerateAfter a 1000 mg dose of valacyclovir, cimetidine lowers the kidneys' clearance of aciclovir (the active form the body makes from valacyclovir) and raises the AUC of aciclovir by about 25%. Probenecid does the same thing but raises the AUC by about 45%. Both work by blocking the active release of aciclovir through the kidneys.
These medicines are often taken together with some care. Your doctor may change a dose, space them apart or check on you more often. Make sure your doctor and pharmacist know you take both.