May be affected by CYP3A4 inducers or inhibitors. Reduced therapeutic effect w/ phenytoin, barbiturates, primidone, carbamazepine, rifampicin & possibly oxcarbazepine, topiramate, felbamate, griseofulvin & products containing St. John's wort. Increased or decreased plasma conc w/ HIV/HCV PIs & NNRTI. Increased plasma conc w/ azole antifungals eg, itraconazole, voriconazole, fluconazole; verapamil; macrolides eg, clarithromycin, erythromycin; diltiazem & grapefruit juice. May influence results of certain lab tests including biochemical parameters of liver, thyroid, adrenal & renal function, plasma levels of carrier proteins eg, lipid/lipoprotein fractions, parameters of carbohydrate metabolism & coagulation & fibrinolysis.