Monitor Closely (1)pentobarbital will lessen the level or influence of fentanyl by affecting hepatic/intestinal enzyme CYP3A4 metabolism. Modify Therapy/Keep track of Carefully. Coadministration of fentanyl with CYP3A4 inducers could lead on to a decrease in fentanyl plasma concentrations, lack of efficacy or, probably, enhancement of the withdrawa