On the face of it, the sheriff's got a point: it's illegal under 18 USC §922 (g)(3) to possess a firearm if one is "an unlawful user of or addicted to any controlled substance." Everyone who's filled out an ATF form 4473 is familiar with that particular question (question 11.e.
http://www.atf.gov/forms/download/atf-f-4473.pdf).
But here's the problem: as far I can make out, the Controlled Substances Act prohibits manufacturing, distributing, dispensing and possessing controlled substances, but it does not prohibit their
use. To the best of my knowledge, the default drugs charge is possession. If so, then there is, under federal law, legally no such thing as an "unlawful user."
If there were, incidentally, that would make the aforementioned question 11.e. on the ATF form 4473 a flagrant violation of the Fifth Amendment right against self-incrimination.
And if there is no such thing as an "unlawful user," you can't be one, and thus you can't be disqualified from possessing a firearm.