Observe Closely (1)pentobarbital will lessen the extent or influence of fentanyl by influencing hepatic/intestinal enzyme CYP3A4 metabolism. Modify Therapy/Observe Intently. Coadministration of fentanyl with CYP3A4 inducers could lead on to a lower in fentanyl plasma concentrations, deficiency of efficacy or, perhaps, growth of a withdrawal syndrom