I have no doubt there is such a thing a demon possession, and I have encountered it on several occasions. I also know that the condition is often given a psychiatric terminology. On the other hand there are Christians who wrongly name all sorts of physical conditions demonic possession.
The big thing is not to be afraid of demonic powers. They are defeated
and totally subject to the Son of God. I believe genuine possession responds
to prayer in the name of Jesus. On the other hand there are some people
my wife and I know who are diagnosed as schizophrenic (hear voices, and
are disturbed in other ways), and they do not seem to respond quickly to
prayer. If there is no immediate healing, I think it is important to have
the person checked by a physician as soon as possible.