Check Closely (1)phenobarbital will lower the level or result of fentanyl transmucosal by impacting hepatic/intestinal enzyme CYP3A4 metabolism. Modify Therapy/Check Carefully. Coadministration of fentanyl with CYP3A4 inducers may lead to your lower in fentanyl plasma concentrations, not enough efficacy or, perhaps, advancement of a withdrawal synd