pentobarbital will lower the extent or result of irinotecan by affecting hepatic/intestinal enzyme CYP3A4 metabolism. Use Caution/Observe. pentobarbital will lessen the level or outcome of fentanyl by influencing hepatic/intestinal enzyme CYP3A4 metabolism. Modify Therapy/Keep track of Intently. Coadministration of fentanyl with CYP3A4 inducers