You cannot take a controlled substance or prescription medication without a prescription from a licensed practitioner.
You are not qualified if you use a controlled substance identified in 21 CFR 1308.11 Schedule I, an amphetamine, a narcotic or any other habit-forming drug.
There is an exception. The prescribing doctor can write that you are able to operate a commercial vehicle safely while taking the medication. In this case, the medical examiner may, but does not have to, certify the driver.
Note: Any medication used for the prevention of seizures is disqualifying.