Keep an eye on Closely (one)phenobarbital will reduce the extent or influence of fentanyl transmucosal by influencing hepatic/intestinal enzyme CYP3A4 metabolism. Modify Therapy/Check Closely. Coadministration of fentanyl with CYP3A4 inducers could lead on to your lower in fentanyl plasma concentrations, deficiency of efficacy or, quite possibly, a