Cannabis

To get hired you had to pass a test. If you failed the test, you could deny the results and not have the test set in to the lab so you could still retest in 6 months, or you could have it sent into the lab. If it was clear, you're fine, and you get the job. If not, that's a confirmed result and you can't be hired there permanently. The lab would eliminate the false negatives.

Drug testing (particularly for marijuana) is not a good thing, though. Oppressive as hell, and don't mean shit. As my dad says, "I could be an alcoholic who beats his wife and kids every night, and as long as I come to work sober in the morning, they don't care. But smoke a single bowl with someone in a legal state and that's grounds for getting fired."