In Kurt Koch's Occult Bondage and Deliverance, Dr Lechler lists some points of distinction between demonic possession and mental illness. (The following agree with our observations at Fitrah Centre).
Although the possessed person may be restless and driven into a rage at times, he still remains sane in his thoughts, whereas one with mental problems does not.
The mental patient will speak in extravagant tones of demons which he alleges to be living inside of him. However, the possessed person avoids all mention of demons as long as no one approaches him on a spiritual level.
The voices which a mental patient claims [to] originate from strange people are usually of a pathological nature. The satanic voices heard by a possessed person are of a completely different nature. If the voices are demonic in origin, they will attempt to lure the person away from God, whereas if they are the result of some natural abnormality, they will speak about unnatural and nonsensical things.
Schizophrenics who continually talk about being possessed are deluding themselves. On the contrary, a person who is really possessed will never let the idea of possession enter his head, even if there is no other logical explanation for his condition.
The person whilst in a state of possession blasphemes without the slightest remorse. The mental depressive will lament the fact that he thinks such things and will try not to express them. The above five points are an excerpt from 'The Exorcist Tradition in Islam' by Abu Ameenah Bilal Philips.
The possessed person clearly can be seen to have two different identities & personalities simultaneously, especially when the devil attacks them —like strangling, scratching or expressing the intent to harm— whilst at the same time the patient attempts to fight and resist the attack and pleads for assistance from family/experts around them during these violent episodes. On the other hand, the mental patient does not attack himself and resist the attack simultaneously, or plead desperately for assistance against their own attacks.
A patient may suffer from both demonic possession and mental illness and therefore exhibit a mixture of symptoms. In such cases, the family of the patient must ensure the patient receives both spiritual [ruqya] and mental treatment administered by the respective experts.