Could I fail a drug test for a prescribed substance?Let me specify narcotic substance,for a prescreened test?

As a retired nurse epidemiologist I can let you know that depending on your employer, the lab and exactly what the employer wants tested, you could be looking at giving a sample of urine, blood draw, and having a lock of hair clipped at least 2 inches long. The lab order form is formatted specifically for the company which can test for Cl One drugs (all illegal, even marijuana) and Cl Two drugs (demerol, morphine, oxycodone, etc.) By the way, if you have been to a party where people were smoking marijuana, inhaling second hand smoke will show up in your body. If you go to anyplace where there is illegal smoking, turn right around and walk back out. Medical professionals don't listen to any body's story of why their test come back positive. Before any test is done, you fill out forms and on those forms you list any drugs you've taken over the past 2 or 3 months prescription or over the counter.