Just another point to ponder: years ago I failed not one, not two, but three pre employment drug screens. Each time I failed the on site and when the sample was sent out I failed still but for some different reason requiring a retest. Each time I talked with the testing agency and begged to repeat, stating is never failed a test before. Finally, after the third test going out, they realized it was a combination of medications I was on, the fact that my A1C was crazy high at the time, and that I have mildly impaired liver function from an illness. This was more than 10 years ago, so I don't remember the substances I supposedly kept failing for, but sometimes, I know because it's happened to me, you fail for medical reasons you may not even be aware of.