I'm torn over this.
Part of me feels that people rely too much on medication to get through bumps in the road. Bad shit happens all the time, it doesn't necessarily require a trip to the pharmacy to get through it.
That part of me is ok with over-medicated people losing their right to own a gun. If you can't cope, I'm not sure I want you in a position where you have to make the fight/flight decision.
Now for people that truly need medication to make them stable, I can't say that I'm against them not having access to guns. My thoughts go to the Auroura shooter who was losing it, and he still could get what he needed legally. This doesn't address Sandy Hook at all.
But it falls apart because I don't trust the government to extend it's reach, and I don't trust that they will do this in a sane manner.