The FMCR book states that in the event of a positive drug test, pre employment or random, the MRO (medical review officer) will contact you and ask if you are on a prescription, the doctor that gave you the scipt must know what you do for a living and all that, it has to be prescribed for a medical condition, obviously it can't be for a condition that prevents you from doing your job, it doesn't matter what you are on when you get your cdl as long as you do it on your own, they don't drug test when you obtain your licence. Companies have to give you a pre employment drug screen and random drug and alcohol per dot rules. I do think that there are doses that you can and can't drive on though, my cousin had back surgery and was on some sort of pain pill and he said that his dose was legal to drive on, any more and he couldn't. Bottom line, if you have it on a legal prescription you are ok, you will have to prove it but don't drive on it, that would be my take, hope this helps answer your question.
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Narcotics/CDL 
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