Observe Intently (1)phenobarbital will lessen the level or effect of fentanyl transmucosal by impacting hepatic/intestinal enzyme CYP3A4 metabolism. Modify Therapy/Observe Intently. Coadministration of fentanyl with CYP3A4 inducers could lead into a decrease in fentanyl plasma concentrations, not enough efficacy or, potentially, growth of the withd