Monitor Carefully (1)phenobarbital will decrease the level or effect of fentanyl transmucosal by impacting hepatic/intestinal enzyme CYP3A4 metabolism. Modify Therapy/Observe Closely. Coadministration of fentanyl with CYP3A4 inducers may lead to some lessen in fentanyl plasma concentrations, insufficient efficacy or, probably, improvement of a with