Watch Carefully (1)phenobarbital will lessen the extent or outcome of fentanyl transmucosal by impacting hepatic/intestinal enzyme CYP3A4 metabolism. Modify Therapy/Monitor Carefully. Coadministration of fentanyl with CYP3A4 inducers may lead to some reduce in fentanyl plasma concentrations, insufficient efficacy or, potentially, progress of a with