primidone will decrease the extent or effect of diazepam by affecting hepatic/intestinal enzyme CYP3A4 metabolism. Use Warning/Observe.phenytoin will reduce the extent or result of diazepam by impacting hepatic/intestinal enzyme CYP3A4 metabolism. Use Caution/Observe.Valium is detectable in blood for 6 to 48 hrs. Blood tests are generally made use