Kochmann, you are aware of studies such as Janowski et al 1972?
A study which showed how Amphetamines (which coincidently raises the levels of dopamine_ gave non-schizophrenics symptoms similar to schizophrenia, and exacerbated the schizophrenic's symptoms).
Or perhaps Timmons & Hamilton 1990 which further dwelved into amphetamines and reported that those that took them developed an acute form of psychosis called "Amphetamine psychosis". This psychosis' symptoms mirrored those of schizophrenics, even more similar was that a drug known as Chlorpromazine a drug used to treat schizophrenia, (an anti-psychotic) was also used to some success in treating the psychosis.
Continuing with Chlorpromazine, Barlow & Durrand 1995 found a 60% sucess rate in reducing (not entirely sure if it totally removed) some of the symptoms of schizophrenia (such as hallucincations and delusions) when using the drug.
These three studies give strong evidence for the chemical side of the debate.
So, as Azzaman above has said: Where's your evidence that Demonic Possession causes mental illnesses?