To support Aliento's point about false positives, anyone with a relatively simple knowledge of biology/chemistry would be able to create false negatives; there are certain painkillers that you can take in combination - some of them controlled substances, some not - that will create a negative result on a test for those drugs (in case you're interested in the biology, it's by competitive inhibition).
Not only that, but very innocuous things can also create false positives, such as poppy seeds; as they contain very trace amounts of the same chemicals in opium, morphine and heroin (for obvious reasons), they can trigger a positive response. And, obviously, prescription drugs can trigger positive results for their more illicit counterparts.
I say that unless a test can be proven to be 100% effective against a false positive, then they shouldn't be used. to be accused of dabbling in illicit drugs - based on a faulty test - and thereby denied a job or even fired seems inexcusable on a moral basis.