How can you fail a drug test if you have the drug in your system?

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improved answer- you can pop positive with a false positive (you are not on drugs) only if the company paying for the drug screening test does not want to pay for the confirmation test (mass spectrometer). If they do pay for the mass spectrometer you will not have a false positive no matter what your taking/eating.

For example: With the mass spectrometer an experienced tester can tell the difference between you getting pot smoke blown in your face and you actually smoking it. This is not to say that just because you caught a contact you will pass the drug test, it depends on the tester and it depends on the company paying for the test. Zero tolerance is zero tolerance after all.