- At least 2 weeks since prior drugs specifically known to interact with valproic acid including, but are not limited to, aspirin, felbamate, rifampin, amitriptyline/nortriptyline, carbamazepine, clonazepam, diazepam, ethosuximide, lamotrigine, phenobarbital, primidone, phenytoin, tolbutamide, warfarin, or zidovudine