Increased plasma conc by strong CYP3A4 (eg, ketoconazole, itraconazole, voriconazole, ritonavir, clarithromycin & telithromycin) & CYP2A6 (eg, methoxsalen) inhibitors. Decreased plasma conc by tamoxifen & CYP3A4 inducers (eg, phenytoin, rifampicin, carbamazepine, phenobarb & St. John's wort). Altered systemic serum conc of drugs mainly dependent on CYP2C19 elimination & w/ narrow therapeutic index (eg, phenytoin, clopidogrel).