I heard this comment on the radio by a marijuana activist:

Think of the message drug testing sends to people. It takes 30-90 days for marijuana to naturally leave your system and not show up on a drug test. yet, it takes as little as 24 hours for crack, cocaine and alcohol to leave your system. What this means, in effect, is that you shouldn't smoke a joint on the weekends. Instead, you should smoke some crack and drink booze, because that stuff won't even show up on that drug test you have to take on Monday.

Man, what a crazy world we live in.