Observe Closely (1)phenobarbital will lower the level or influence of fentanyl transmucosal by influencing hepatic/intestinal enzyme CYP3A4 metabolism. Modify Therapy/Observe Carefully. Coadministration of fentanyl with CYP3A4 inducers could lead on to a decrease in fentanyl plasma concentrations, insufficient efficacy or, potentially, progress of