RealmMan;171230
Additionally, Venkman's a Ph. D, not an MD, so he's not licensed to administer medication. I mean, his ethics are questionable, but not THAT questionable.
But wouldn't Thorazine be involved in a Psychologist's practice? Maybe not directly, but he certainly could have got his hands on some.
But yeah, as Gamora says….it was just used to advance the plot. They probably didn't expect to have a group of fanboys discussing the dialogue 25 years on.