Unexpected bleeding or decreased effect w/ primidone, phenytoin, barbiturates, carbamazepine, oxcarbamazepine, topiramate, felbamate; rifampicin; ritonavir, nevirapine; penicillins, tetracyclines, griseofulvin; St. John's wort. Increased levels w/ ketoconazole; cimetidine; verapamil, diltiazem; erythromycin; antidepressants; grapefruit juice. May influence effect of lamotrigine.