Studies Assess Whether Cannabis Use Is Causally Linked to Schizophrenia

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."