Keep track of Intently (1)phenobarbital will lower the level or impact of fentanyl transmucosal by influencing hepatic/intestinal enzyme CYP3A4 metabolism. Modify Therapy/Watch Closely. Coadministration of fentanyl with CYP3A4 inducers could lead to a decrease in fentanyl plasma concentrations, deficiency of efficacy or, possibly, enhancement of th