Keep track of Carefully (1)phenobarbital will reduce the extent or impact of fentanyl transmucosal by impacting hepatic/intestinal enzyme CYP3A4 metabolism. Modify Therapy/Watch Closely. Coadministration of fentanyl with CYP3A4 inducers could lead on to some lessen in fentanyl plasma concentrations, deficiency of efficacy or, maybe, development of