I would be curious where you are getting the better than 50/50 chance, or do you mean eventually? My psychiatrist informed me that the average person will try between 3 and 7 different drugs before finding one that both controls the symptoms of whatever ailment they are working on, and has side effects the individual finds manageable. I am on my 6th such, and it took 3 pretty bad experiences to find the first one that worked for me, that one lasted a while, then stopped working, the next lasted a few months. My current med has been working well for over a year so I am hopeful, but transitioning between meds can be very hard, and there is no guarantee how long it will work. Obviously it is different person to person, but that is kind of the point.
And to bring it back to topic, I think anyone with an active prescription should go through a more thorough background check for purchasing a gun. I know that I had a time when I gave all my guns (not very many, couple of handguns, couple of rifles, shotgun, etc.) to my dad for safekeeping while I worked through my main issues and found a med that worked for me.
I'm sorry. Let me state it out more specifically:
There's a much better than 50/50 chance that the medication you're given will not cause you to lose control and shoot up a school.
Every medicine I've tried (admittedly, I'm not on my 7th) I've felt different. Only once with one medication did I feel any truly disturbing side effects(enhanced aggression). The rest all had a positive effect on me, and although not all were "the perfect drug for me", did not make my life worse.