- Are currently being treated with agents that are metabolized by cytochrome P450 (CYP)3A4 with a narrow therapeutic margin (for example, alfentanil, cyclosporine,diergotamine, ergotamine, fentanyl, pimozide, quinidine, sirolimus, and tacrolimus) or CYP2B6 (for example, cyclophosphamide, ifosfamide, tamoxifen, efavirenz, propofol, methadone, and bupropion)