pentobarbital will lower the extent or impact of fentanyl transdermal by affecting hepatic/intestinal enzyme CYP3A4 metabolism. Modify Therapy/Keep an eye on Intently. Coadministration of fentanyl with CYP3A4 inducers could lead on into a lower in fentanyl plasma concentrations, lack of efficacy or, probably, growth of the withdrawal syndrome insid