Keep an eye on Closely (1)pentobarbital will reduce the extent or effect of fentanyl transdermal by influencing hepatic/intestinal enzyme CYP3A4 metabolism. Modify Therapy/Observe Intently. Coadministration of fentanyl with CYP3A4 inducers could lead on into a lower in fentanyl plasma concentrations, insufficient efficacy or, perhaps, growth of the