The problem being is that i dont think they really care what you do outside work as long as its not in your bloodstream therefore causing you to suffer the effects of the drugs the next day. As far as i know certain drugs can stay in the bloodstream for quite a bit of time even if you have stopped. Marajuana if i recall correctly can stay in the bloodstream for quite a few hrs, think its something like 8 hrs or more and thats after you have stopped inhaling it. But again i could be talking out of my ass as i cant really remember tbh
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Most probably this company is trying to limit the potential risk of drug taking and its effect on its workforce and their workload i think.