Working in a psychiatric ward you will very often see patients with a first onset of paranoia or other psychotic symptoms after a chronic use of cannabis. A recent research paper discussed the following hypotheses for a possible relationship:
There is a causal relationship between cannabis use and schizophrenia (psychotic disorders).
The cannabis use precipitates schizophrenia in vulnerable persons.
Cannabis use exacerbates schizophrenia -> symptoms are more severe.
Patients with psychotic disorders are more liable to abuse cannabis
This Australian study found a rise in the prevalence of cannabis use and a decrease of the age at the beginning of regular cannabis consumption over the last 30 years. They found no clear increase of the prevalence of schizophrenia in Australia in this time span. We know that cannabis abuse is rather common among patients with schizophrenia, maybe sometimes a rather bad attempt to cope with psychotic symptoms or irritability or hallucinations.
So the authors of this study think that cannabis use is not the main causal factor for the incidence of schizophrenia, but seems to precipitate the onset of psychotic symptoms / paranoia for vulnerable persons. So if you have a high vulnerability for schizophrenia the abuse of cannabis will most likely lead to severe paranoia and a worse outcome!
The co-occurrence of substance abuse and schizophrenia is one of the worst problems in psychiatry! It is very difficult to offer a good treatment approach for this group of patients. Special treatment programs are available for schizophrenic patients with cannabis dependence or other types of substance abuse.