How long can clean pee be stored for a drug test?

I would not do that, if I were you. Using someone else's urine to pass a drug test is very much illegal, and if you get caught, the penalty will be far, far worse than being fired from your job. You might very well end up in jail. And then your whole career will be completely and permanently ruined (as nobody wants to hire an ex-convict, except maybe McDonald's).

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