I think you can be on any number of prescribed drugs (with subscription) and nutty as a fruitcake in a doctor’s opinion, but unless you’ve been adjudicated mentally unfit, it doesn’t prohibit purchase from an FFL dealer.
Also, about the only gun control law I'm in favor of is to have those on certain SSRIs or psychological drugs be put on temporary (or permanent) NICS ban. Once the patient sees the MD and a PharmD fills the script, should go right to NICS.
Bi polar and on lithium? No gun for you.
Taking something for Schizophrenia? Nope.
Think you are a man when you have a vagina? Too mentally ill for me to give you a gun.