pentobarbital will lower the extent or impact of tamoxifen by impacting hepatic/intestinal enzyme CYP3A4 metabolism. Use Warning/Observe.Monitor Closely (one)pentobarbital will minimize the extent or impact of fentanyl by affecting hepatic/intestinal enzyme CYP3A4 metabolism. Modify Therapy/Watch Closely. Coadministration of fentanyl with CYP3A4 in