Check Intently (1)phenobarbital will reduce the extent or outcome of fentanyl transmucosal by affecting hepatic/intestinal enzyme CYP3A4 metabolism. Modify Therapy/Observe Closely. Coadministration of fentanyl with CYP3A4 inducers may lead to a lessen in fentanyl plasma concentrations, insufficient efficacy or, perhaps, progress of a withdrawal syn