Several of you are forgetting that LAWFUL drug use is allowed. Whether it's nicotine, caffeine or alcohol all are legal to use. Obviously, their misuse (drunk driving for example) is unlawful.
While some states have decriminialized medical use of marijuana, it REMAINS illegal under FEDERAL LAW. While Obamas Justice Department says they aren't going to pursue those using "medical marijuana" they COULD- because it's illegal.
Question 11e on Form 4473 asks:
"Are you an unlawful user of, or addicted to, marijuana or any depressant, stimulant, narcotic drug, or any other controlled substance?"
You can dance around it all day that it's doctor prescribed, it's an herb, a natural substance, blah, blah, blah- if it's illegal under Federal law you cannot truthfully answer question 11e on the Form 4473.
Get caught lying on a Form 4473 gets you up to 10 years imprisonment and/or up to a $250,000 fine.