i learned about it from a friend who was tested and smoked more than me. he would take a bigger risk than i would allow myself by smoking up untill the last few days before a test and he would pass. i had more to loose by being dirty, so i would only smoke one week a month right after my test then clean out with niacin. it would show small amounts of thc in my system, but it honestly did that weather i smoked or not. once it had been months since i had done anything and it showed a little positive. i am not a heavy smoker and havnt been for a long time(years, since highschool), so i dont know how to explain being even a little positive even when i knew for sure that i shouldnt have any in my system at all.