Yes, I think so. Although I suppose that if it's only people who are currently taking psychoactive drugs then I'd be fine anyway. And, as I say, if you make this the law, then people with mental issues will be less likely to report their symptoms, meaning that they can still get the guns but will not be treated for their illness. If they are suffering a serious psychosis, then I would agree that they should not be licensed to have a gun, but they will probably then have significant other limits to their freedom anyway.
I'm not sure what you mean by mind-altering, anyway.
As in, interrupting or changing the natural function of the person's brain. Though it performs many functions, the brain is a single organ and medicines which affect for instance mood can't
not affect other attributes like judgment. We know for a fact that psychiatric drugs affect judgment because they are packaged and promoted with a very straightforward warning that they're capable of causing hallucinations or "suicidal thoughts or tendencies". Since something like 1 percent of actually using a gun is the motor mechanism of pulling the trigger and the remaining 99% is all about judgment, it defies common sense to legally enable people known to be of compromised judgment to possess and carry guns about with them.
I was under the impression that most of these individuals (the mass shooters) who end up having had mental health issues (which is most to nearly all of them) had these issues since minority. This makes it not a problem, since children are taken to their mental health care providers by their parents, not of their own accord; meaning that it's extremely unlikely that a child will be able to prevent himself from receiving treatment because he wants to own a gun someday. Now again it is my
impression that most of these mass murderers we've been seeing fall into this category; for instance the individual in the present case did; I believe the theater shooter in Aurora had similarly received behavioral treatment in youth as did the Virginia Tech shooter and the man who attempted to kill Congresswoman Giffords and succeeded in killing many bystanders in that incident (I could be misinformed, I'll freely admit; but these are my understanding of the data at this point). Certainly not all such situations could've been prevented by this kind of measure, but a good many of them easily could have. There's no reason why we have to wait and hold out for "One Measure To Combat Them All" because certainly no such thing can possibly exist.
As for those who developed their mental problems while adults; while I concede that I do think someone choosing not to accept mental health services because he wants to be able to own a gun is pretty much exactly the kind of person who shouldn't be let anywhere near guns, we have to also consider that this is an unquantifiable eventuality; we have no way of actually knowing whether anyone would in fact make such a judgment, or how many such people would. The currently-existing "stigma" on mental illness has not stopped millions from seeking treatment (or being taken to seek treatment by their parents) thus far. To me it seems to be a case of a bunch of people who
may possibly have compromised judgment carrying guns versus individuals who can be objectively determined with a reasonable degree of certainty to have compromised judgment (because we know they're taking a substance which compromises judgment).
I have no doubt that some peoples' feelings may be hurt because taking Abilify or Ritalin disqualifies them from owning firearms; but I think that's a pill some of us (myself included, by the way, just for reference) may just have to swallow.