Your employer can refuse to hire you, or retain you if already hired, if you fail the drug test due to medical marijuana. Marijuana is still an illegal substance according to federal law, and an employer can refuse to employ you for using an illegal substance even if you have a prescription for it. Good luck, but you will likely not pass the test.
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I think you are misinformed about how the test reporting will go. The testing company is hired and paid by the employer. They will report the results of the test to the employer. I have handled many matters of pre-employment testing. I have never seen one where the testing company contacted the tested person and discussed the circumstances and facts underlying the test result.
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