- Concomitant use of narrow therapeutic index drugs that are metabolized by CYP3A4 (i.e. alfentanil, cyclosporine, dihydroergotamine, ergotamine, fentanyl, pimozide, quinidine, sirolimus, tacrolimus), CYP2C9 (phenytoin, warfarin), and CYP2C19 (S-mephenytoin). [Note: Patients on stable doses of anti-coagulation with warfarin and fentanyl will be eligible, as long as they are monitored closely with additional INR monitoring].