“Drugs” are a pretty broad category of thing, it’s unlikely that you’re ever going to get an honest answer out of anyone, and it’s even more unlikely that anyone using illegal drugs who wants a gun is going to not buy a gun because of this. It’s just more war on drugs bullshit.
I’ll also add that if people were actually worried about the competency of people buying guns to own guns, then it would make far more sense to put the burden of proof on the person buying to prove their competency. But that’s not what this law is really about.