pentobarbital will lower the extent or impact of guanfacine by impacting hepatic/intestinal enzyme CYP3A4 metabolism. Modify Therapy/Check Carefully. Solid or average CYP3A4 inducers noticeably lessen guanfacine plasma concentrations and elimination 50 percent-life.pentobarbital will decrease the extent or outcome of methadone by impacting hepatic/