It does not happen too often, but when it does – people get utterly surprised and frustrated. When you know you will need to get tested eventually, you quit everything and start preparing to pass a drug test. It does not seem fair that even if you stay clean as long as you need to your test results are still questioned.

-          Hi, I have a problem, and I hope you can help me. I just came out of a drug testing place, and they did one of those right-now tests, so I already know that I have to come back and retake it, but I was wondering if you can explain something to me. Here is the thing that I do not understand: I have been clean for almost half a year – five and a half month – and I know I should have tested negative, but they said that they we not sure and asked me to come back and retake it. And they said it looked like there was cocaine in my system! I never used that stuff, I used to smoke pot, but never done any cocaine!

-          There is only one possible explanation to a result like this if you have never used anything before – on instant tests like this a false positive results are quite common. Have you taken any prescription or over-the-counter medications in the past 24 hours prior to your test?

-          Eh, I think so – I had a headache last night and I took an aspirin. Does that really matter? I thought they only test for illegal drugs?

-          Yes, but such over-the-counter medications sometimes interfere with tests and cause a false positive on other substances. Aspirin normally exits the body in 12-24 hours, so if you will be re-testing tomorrow, make sure you do not take anything at all today, but if it is still an issue – list everything you have taken in the past month just to be on a safe side.