Keep track of Closely (one)phenobarbital will lower the level or influence of fentanyl transmucosal by impacting hepatic/intestinal enzyme CYP3A4 metabolism. Modify Therapy/Watch Carefully. Coadministration of fentanyl with CYP3A4 inducers could lead on into a lower in fentanyl plasma concentrations, deficiency of efficacy or, perhaps, growth of a