Check Closely (one)oxcarbazepine will lessen the level or effect of fentanyl by affecting hepatic/intestinal enzyme CYP3A4 metabolism. Modify Therapy/Keep an eye on Carefully. Coadministration of fentanyl with CYP3A4 inducers may lead into a decrease in fentanyl plasma concentrations, lack of efficacy or, potentially, development of a withdrawal sy