Den,
When you go looking for answers to questions like yours above, there are many answers out there. I am going to do my best to give you the information that I've found with the appropriate links to that information.
Here are some questions and answers that I found on the Department of Transportation website:
Question:
Can a CMV (commercial motor vehicle) driver be disqualified for using a legally prescribed drug?
Answer:
Although the driver has a legal prescription, he/she may be disqualified if the mediation could adversely affect the driver's ability to drive a CMV safely.
Question:
Is there a list of prohibited drugs for being medically qualified to drive a commercial motor vehicle (CMV)?
Answer:
Section 391.41(b)(12) states: A person is physically qualified to drive a CMV if that person does not use a controlled substance identified in 21 CFR 1308.11, Schedule I, an amphetamine, a narcotic, or any other habit-forming drug. Exception: A driver may use such a substance or drug, if the substance or drug is prescribed by a licensed medical practitioner who is familiar with the driver’s medical history and assigned duties; and has advised the driver that the prescribed substance or drug will not adversely affect the driver’s ability to safely operate a CMV. This exception does not apply to methadone.
Medical Examiners are required to give careful consideration to the effects of medications on a driver’s ability to operate a CMV safely before rendering the driver qualified
Question:
Can a driver be certified if he is taking Methadone?
Answer:
No
http://www.fmcsa.dot.gov/rules-regulations/topics/medical/faqs.aspx
I have sent an email to the DOT to insure the information listed on the site is current and up to date and I will let you know when I hear something back from them.
Have a good day,
Dee