Keep track of Carefully (1)phenobarbital will lessen the extent or influence of fentanyl transmucosal by impacting hepatic/intestinal enzyme CYP3A4 metabolism. Modify Therapy/Watch Closely. Coadministration of fentanyl with CYP3A4 inducers could lead on into a decrease in fentanyl plasma concentrations, not enough efficacy or, perhaps, development