Monitor Closely (1)phenobarbital will decrease the level or influence of fentanyl transmucosal by influencing hepatic/intestinal enzyme CYP3A4 metabolism. Modify Therapy/Observe Closely. Coadministration of fentanyl with CYP3A4 inducers could lead on into a reduce in fentanyl plasma concentrations, lack of efficacy or, maybe, improvement of a withd