Decreased efficacy w/ phenytoin, phenobarb, primidone, bosentan, carbamazepine, rifampicin, oxcarbazepine, rifabutin, topiramate, felbamate, griseofulvin, ritonavir, nelfinavir, efavirenz, St. John's wort. Increased or decreased plasma conc w/ HIV PIs (eg, nelfinavir), NNRTIs (eg, nevirapine), HCV medicinal products (eg, boceprevir, telaprevir). Increased serum conc w/ strong (eg, ketoconazole, itraconazole, clarithromycin) or moderate (eg, fluconazole, diltiazem, erythromycin) CYP3A4 inhibitors. Reduced steroid absorption & efficacy w/ medical charcoal. Increased plasma & tissue conc of ciclosporin. Decreased plasma & tissue conc of lamotrigine.