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Niacin is supposed to help if you do buy/steal some make sure its not "flush free" or time release.
Otherwise green tea water and cranberry juice.
(Niacin may also be in some of your multivitamins laying around worth checking out)
a week to a month or 2 depending on frequency of use. Not doing drugs, as already mentioned, makes them pretty worry-free though. If you're looking for a job soon, just stay away from the ganj. If you're parents are piss testing you, first off they're fucking retards, second you'll find a way to cheat that test.
Well even though you didn't want to buy anything. Go too walgreens or any pharmacy and buy two drug tests. Then drug test yourself once you get home if you pass your set. If you fail start drinking water, tea, piss alot, possible niacin or whatever flushes out your system. Then once after all that try the test again. If you fail repeat.