Is it Illegal to be on Prescription Drugs and Carry a Gun?

I heard through the grapevine that a CCW instructor was telling people it was illegal for them to have a gun and prescription medication in their possession at the same time. I’ve heard others say it’s illegal for gun owners to take anti-depressants or other medicals for psychological problems, pain medications, sleeping pills, etc. This is false, except if you are taking them illegally without a prescription.
 
Specifically, federal law says “who is an unlawful user of or addicted to any controlled substance," 18 USC § 922(g). An "unlawful user" is someone who is an addict or using a prescription drug without a valid prescription. Have a prescription and you're not a pill popper? You're good.
 
21 USC. § 802 defines an addict and the drugs that are illegal.:

The term 'addict' means any individual who habitually uses any narcotic drug so as to endanger the public morals, health, safety, or welfare, or who is so far addicted to the use of narcotic drugs as to have lost the power of self-control with reference to his addiction.

Sounds like the people that the pharmacist get suspicious of, right? 

​The DEA has the short version of the scheduled drugs. Schedule 1 drugs—marijuana, cocaine, heroin—are always off-limits federally, though as we know, weed is no longer a big deal socially, the ATF takes marijuana and guns seriously. Prescription drugs (Schedule 2-5) are not illegal, when taken by someone who has a valid prescription.
 
So no, if you take Xanax for anxiety off your doctor’s prescription, you are not committing a crime if you carry or buy a gun. Where it becomes problematic is if you are taking the drugs without a prescription or are obviously abusing them. A medication addict is obvious; you have to go to different doctors all the time to get prescriptions for Oxy or whatnot because you’ll go into withdrawals without it. Taking your Xanax everyday simply to avoid your hellish job jacking your blood pressure up so high you need medication for that is not addiction.
 
If you used drugs in the past, but have given them up (well and truly sober; not the “I haven’t used in a long time,” which means five minutes ago, lie seen nightly on Cops), you’re okay. Here’s what the ATF had to say about that:

Such use is not limited to the use of drugs on a particular day, or within a matter of days or weeks before, but rather that the unlawful use has occurred recently enough to indicate that the individual is actively engaged in such conduct.

An inference of current use may be drawn from evidence of a recent use or possession of a controlled substance or a pattern of use or possession that reasonably covers the present time, e.g., a conviction for use or possession of a controlled substance within the past year, or multiple arrests for such offenses within the past five years if the most recent arrest occurred within the past year.

​So to summarize: if you have a valid prescription for a legitimate problem and are not addicted to pills, it’s legal for you to own, buy, and carry a gun. If you abuse medication, use medication that is not prescribed to you, or use hard drugs (including marijuana), you are a prohibited person. Having your Xanax in your purse next to your gun is not illegal, but you should probably find some non-medicinal or non-“herbal” remedies for your problems.