Drug testing employees without a significant reason (ie, Health workers) is the most small minded and self destructive form of idiocy known to management-kind.
It's none of the company's business what you do on your time. If you fuck up, they can fire you for incompetence. If they suspect you're a drugged up knife wielding lunatic, well there will be more useful evidence from the corpses and screaming innocents around the water cooler. I guess then they can call the cops.
Either way. Screw them.
and no I have never taken one, let alone failed. In case you wonder.