Richmond, VA: Several recently published studies provide further insight regarding whether cannabis exposure is an independent contributor to schizophrenia in non-predisposed subjects.
The first paper, published in The American Journal of Psychiatry, assessed the association between substance abuse, psychosis, and the progression to schizophrenia in a Swedish national sample. Authors reported that individuals with a substance-induced psychotic disorder typically possessed elevated familial risks for both drug misuse and psychosis. Cumulative risk for schizophrenia following substance-induced psychosis was 11 percent, with the strongest association identified among those who had experienced cannabis-induced psychosis. Nonetheless, researchers concluded, "Schizophrenia following substance-induced psychosis is likely a drug-precipitated disorder in highly vulnerable individuals, not a syndrome predominantly caused by drug exposure."
The second paper, published in the journal Addiction, assessed the association between schizophrenia and substance abuse in a cohort of over 3.1 million Danish adults. They reported, "A diagnosis of schizophrenia was positively associated with the risk of developing substance abuse. ... [S]chizophrenia [was] primarily associated with an increased risk of abuse of cannabis, alcohol, stimulants, and other substances. The association was still significant 10-15 years subsequent a diagnosis of schizophrenia."
They concluded, "[A] diagnosis of schizophrenia is significantly associated with increased risk of subsequent diagnosis of substance abuse" – a finding that is consistent with prior studies.
Finally, a newly published review paper in the journal Current Psychiatry Reports states: "The evidence for cannabis acting as a causal factor in schizophrenia has so far not been established. ... Overall, we still have insufficient information and knowledge about who is at risk of developing cannabis psychosis prior to cannabis exposure to reliably produce a public health prevention strategy."
Full text of the study, "Prediction of onset of substance-induced psychiatric disorder and its progression to schizophrenia in a Swedish national sample," appears in The American Journal of Psychiatry. Full text of the study, "Schizophrenia is associated with increased risk of subsequent substance abuse diagnosis: A nationwide population-based register study," appears in Addiction. Full text of the study, "Cannabis and psychosis: Are we any closer to understanding the relationship," appears in Current Psychiatry Reports. Additional information is also available in the NORML white paper, "Cannabis, Mental Health, and Context: The Case for Regulation."