Keep an eye on Intently (1)phenobarbital will lessen the level or result of fentanyl transmucosal by impacting hepatic/intestinal enzyme CYP3A4 metabolism. Modify Therapy/Check Carefully. Coadministration of fentanyl with CYP3A4 inducers could lead to the lower in fentanyl plasma concentrations, insufficient efficacy or, possibly, improvement of a