Watch Carefully (1)phenobarbital will reduce the extent or result of fentanyl transmucosal by impacting hepatic/intestinal enzyme CYP3A4 metabolism. Modify Therapy/Monitor Carefully. Coadministration of fentanyl with CYP3A4 inducers could lead to some lower in fentanyl plasma concentrations, deficiency of efficacy or, quite possibly, improvement of