Drug testing is a joke, so many ways to cheat the test and people do it every day. They can also read your prescription drugs from those drug test, it's not just illegal drugs they're looking at.
I have been denied many things and jobs due to my daily need for a particular drug, side effects read "it might cause drowsiness." How can a drowsy person evaluate appropriately. What that drug test didn't show them is I also have a syndrome that constantly releases Adrenaline. So one takes over and get my life taken care of while the other is only a "might" and never happens to me.
So don't fool yourself that you have nothing to worry about unless you take absolutely no prescriptions and or illegal drugs.