Is it legal to discriminate due to the medicine used to treat a medical condition?

Question

I will present two different scenarios. Lets say Bob has PTSD from his time in the military. He wants to become an FBI agent. Here are the following requirements from the FBI to qualify;

- Must be a U.S. citizen.
- Must be able to obtain a Top Secret clearance.
- Must complete form FD-887, Request for Access to Sensitive Compartmented Information (SCI).
- Must pass an FBI polygraph examination.
- Must pass an FBI-administered urinalysis drug test.
- Must be in compliance with the FBI Employment Drug Policy:
    - No use of marijuana within the last three years.
    - No use of any other illegal drug in the past 10 years.
    - No selling, distributing, manufacturing or transporting of any illegal drugs.
    - No use of a prescription drug or a legally obtainable substance in a manner for which it was not intended within the last three years. 

- Must never have been convicted of a felony.
- Must not be in default on a student loan insured by the U.S. government.
- Must be registered with the Selective Service System (males only, exceptions apply)

Scenario 1) Bob is prescribed a controlled narcotic for his PTSD condition.

Scenario 2) Bob is prescribed medical marijuana for his PTSD condition.

• Is it legal to discriminate employment based off prescribed medicine in either situation?
• If the employer wasn't the FBI, does it change the right to discriminate?

Answer 1

1) Bob could disclose the PTSD condition and seek accommodation for it (in reality, controlled narcotics aren't actually used to treat PTSD but it isn't hard to imagine a situation where another controlled substance, e.g. ketamine, was used to treat this or some other Americans with Disabilities Act recognized disability and the absence of that disability was not a bona fide qualification of the job).

The legal analysis in the case of the FBI (a federal government civilian civil service employer subject to special rules applicable to governmental employers), and a private employer, is not exactly the same, but it ends up in the same place.

2) Medical marijuana is, as a matter of federal law an oxymoron, because it is a Class I controlled substance that as a matter of law (contrary to reasonable facts) has no medical applications, and the FBI is charged with enforcing this law (among other agencies), so medical marijuana would legally disqualify someone from FBI employment.

In Colorado which has legal under state law medical marijuana, employers have been allowed to discriminate based upon medical marijuana use because an employer is at a minimum allowed to treat federal law as enforceable.

It is conceivable that some U.S. state other than Colorado which allows medical marijuana at the state level might reach a different conclusion as a matter of state law on the employment discrimination point, but potentially, the employer could appeal to the U.S. Supreme Court on a pre-emption argument so it would be a tenuous legal position to take.