In the UK Railway industry any staff that do a "safety critical" job (Including office staff) are subject to random drugs screening. Fail and you are dissmissed. No if, buts or maybes. This is supported by all of the relevant unions including the RMT who have to be one of the most militant in the UK!!
However if you tell the company before you get caught they will bend over backwards to help you....... Maybe it's time to introduce this in the Aviation sector....... (retreats to bunker to shelter from incoming fire!!!!)