From what I know, it is very common. It is called prescribing "off label". Pdocs do it either because of supporting research or past experience of success with their own patients. I have MDD and I was briefly on Abilify (an antipsychotic) and Lamictal (a mood stabilizer, usually a medication for bipolar disorder). I have never experienced psychosis and I am not bipolar. Unfortunately neither of these meds worked out for me. Abilify brought me out of the depression for 2 months and then it pooped out. Lamictal interacted with my Xanax XR too much. My former pdoc did want to put me on lithium but I said no way because it causes weight gain and can cause kidney damage down the road. Good luck.