From The Truth About Guns:

As more states increasingly legalize the recreational use of marijuana—24 states according to CBS News currently do—the move has put gun owners who like to toke and can legally do so according to their state’s permittance into a legal conundrum when purchasing a firearm: How to honestly answer question 21(f) on ATF Form 4473 without committing perjury and leave the store with their firearm? The answer is, they can’t.