Serious reply to a serious question.
The jury's out on whether or not cannabis can permanently harm you but the following is what's known at the moment:
1.If you do have to provide a urine sample, drinking about 50 pints of water beforehand will dilute the products to a point where they may not be chemically detected...although you'll spend ages in the little boys/girls room.
2.I don't know if this is used in aviation yet, but your hair will contains a history of your smoking amongst other things. Your hair grows about 1 cm a month and analysis of a single hair can not only reveal what you've been smoking but also things such as what you had for breakfast,dinner, how much you drank etc over the last few weeks.
3.Passive smoking (with regard to cannabis), ie why the Delta guys ran out of the party, is a bit of a misnomer. In order to ingest sufficient cannabis exhalant from someones else's smoking you would have to be in a telephone box with them for about 3 days for it to register on a test.
4.However...please give it up because the idea of a smoker flying the right hand seat of an aircraft I'm in doesn't thrill me.
Whoa..heavy stuff but, as I said, serious points for a serious question.