I think it is common knowledge that substance use and abuse is linked to violence. Many studies have been done showing a link.
Here's one random article:
So -- imagine my surprise when I read this:
Anyone who is voluntarily admitted to a psychiatric hospital, except those solely on the basis of drug or alcohol use, will have to wait six months before being able to obtain a gun permit. The individual will also not be allowed to possess a firearm during those six months.
So, if you have substance abuse - a known risk factor for violence - then you do not have to wait for a gun permit. If you have another mental illness, not linked to violence, then you do have to wait.