The way the question is worded makes it where it has no teeth as a screening implement. No one would admit to it even if they were shooting up heroin in the parking lot. It's questions like that that give the anti's fuel to use against us, mainly that we cry foul when existing laws aren't enforced because they're written in such a way as to be unenforceable. The only way to make the drug use question anything other than a waste of printer ink would be to require a drug test to buy a gun.
If marijuana is made legal and sold the same way booze is there can be no way of tracking its use and no way to screen gun owner based on use of it. But smoking a doobie and going shooting is still a terrible idea.