pentobarbital will decrease the level or effect of phenytoin by impacting hepatic enzyme CYP2C9/ten metabolism. Use Warning/Check.pentobarbital will reduce the extent or result of tramadol by influencing hepatic/intestinal enzyme CYP3A4 metabolism. Use Caution/Observe. Reduced AUC of tramadol as well as the active metabolite (O-desmethyltramadol) w