Keep an eye on Carefully (1)phenobarbital will reduce the level or influence of fentanyl transmucosal by influencing hepatic/intestinal enzyme CYP3A4 metabolism. Modify Therapy/Keep track of Closely. Coadministration of fentanyl with CYP3A4 inducers could lead to some reduce in fentanyl plasma concentrations, insufficient efficacy or, potentially,