I don't know if I have told this story before:
Neither my wife nor I are big fans of pot - sorry to say - very sorry actually, we smoke rather rarely. At the time, both of us were out of work and we were getting a little frightened at the prospect of maybe losing our house. (wait a minute - she was my live in love at the time). She applied at a manufacturing firm with little hope of getting a chance there. One afternoon I pulled out some smoke and we sat in the back yard very baked - warm sun, buzzing giddyness, not caring that we might soon lose the house, that was sort of the point. At any rate, it was a couple three hits of each of some great blond hash.
Sure enough, the next day she gets a phone call. "come in for an interview". She got a little worried but I told her no one ever hires on the spot and we shouldn't get our hopes up anyway. We only had one car, I wanted to be with her (I really like her, even now). We got there, I waited in the car and after only 15 minutes or so she came out with a big frown on her face. I was certain that it was no or at best, we won't know.
Not so. She got in and said "they hired me, contingent on a drug test that I have to take in the morning". I said, go back in there and say that you forgot and you can't make it till the next day, which she did.
All this terror because we, once in months, tried to take the edge off of a difficult situation.
She was handed a questionare when she went in for the test and one of the questions was - have you smoked marijuana in the last month, and she replied yes, then they took her piss.
She went directly to work and we were worried, the test would come back and she would be screwed. After a week, she was called into HR. It seems that the company had a dilema. Her test came back negative but she had admitted that she smoked pot in the last month. It went into a form of internal arbitration and they decided that it didn't matter what she said on the form, the only thing that mattered was the passed test and she kept the job. (wound up being a shitty job but we kept the house)
I have used this knowlege afterward and find that if I smoke only a small amount sporaticly that I have always passed all drug tests. The question is if you are capable of such self control. And of course, if my hair were ever tested I would be completely screwed.