Interesting article on hallucinations by the famous neurologist and author Oliver Sacks over at the New York Times.
I've had small auditory hallucinations (not chemical-induced! :-) ) a few times. The last significant one was a few years ago, when while sitting zazen alone and almost falling asleep I heard a voice say, "I'm not here to defend the bookmobile." I have no idea what that meant or where it came from.
Hallucinations are very startling and frightening: you suddenly see, or hear or smell something — something that is not there. Your immediate, bewildered feeling is, what is going on? Where is this coming from? The hallucination is convincingly real, produced by the same neural pathways as actual perception, and yet no one else seems to see it. And then you are forced to the conclusion that something — something unprecedented — is happening in your own brain or mind. Are you going insane, getting dementia, having a stroke?