It's not necessarily the legality but the requirement, for access to some tiers of classified information, to be drug free for a certain period. The background

It's not necessarily the legality but the requirement, for access to some tiers of classified information, to be drug free for a certain period. The background check required for this access asks, "Have you used [insert contraband] at any time 5/3/2/1 years prior to this application?" The period of prior required abstinence may differ from agency to agency. If I recall, it was 5 years for my IC agency. Of course untruthful answers in the background check, if discovered, would prohibit future federal employment.