Decreased effectiveness w/ phenytoin, barbiturates, primidone, bosentan, carbamazepine, rifampicin, oxcarbazepine, topiramate, felbamate, griseofulvin, ritonavir, efavirenz, St. John's wort. Increased or decreased plasma conc w/ HIV PIs, NNRTIs, boceprevir, telaprevir. Increased serum conc w/ strong (eg, ketoconazole, itraconazole, clarithromycin) or moderate (eg, fluconazole, diltiazem, erythromycin) CYP3A4 inhibitors. Increased plasma & tissue conc of cyclosporin. Decreased plasma & tissue conc of lamotrigine.