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Debunking the Myth of a Link Between Marijuana and Mental Illness

Despite media claims that marijuana can cause psychosis or schizophrenia, there's no science to back it up.

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Investigators stopped short of attributing subjects' cannabis use to the improved outcomes, hypothesizing instead that patients with superior cognitive skills may be more likely to acquire cannabis than subjects with lesser abilities. “[I]t is difficult to determine whether it is cannabis itself that triggers alterations in neuropsychological functioning or if drug-using patients represent a subset of the schizophrenia population who exhibit better neurocognitive performance,” they wrote. Nevertheless, they concluded that it would be reasonable to assume that “cannabis likely has modest … effects on neurocognitive function in schizophrenia.”

Other clinical literature also casts doubt on Large’s claim that marijuana use accelerates mental illness. In a study published last year, a team that included researchers affiliated with the Albert Einstein College of Medicine, Yale University, and the National Institute of Mental Health assessed whether lifetime pot use was associated with an earlier age of onset of symptoms in schizophrenic patients. They c oncluded, "Although cannabis use precedes the onset of illness in most patients, there was no significant association between onset of illness and (cannabis use) that was not accounted for by demographic and clinical variables.”

The researchers also criticized the findings of previously published studies that purported to have uncovered a ‘pot trigger’ for mental illness. “Previous studies implicating cannabis use disorders in schizophrenia may need to more comprehensively assess the relationship between cannabis use disorders and schizophrenia.”

Unlike Earleywine, however, the researchers in this study were not convinced that a large percentage of schizophrenic patients are ‘self-medicating’ with pot. “We … found that about half of our subjects discontinued the use of cannabis when their psychotic symptoms worsened,” said Dr. Serge Sevy of the Zucker Hillside Hospital, who led the study.  “(But) unfortunately, our study did not include questions about (patients’) reasons for using or discontinuing cannabis.  I cannot provide the percentage of patients who discontinued cannabis use because of a worsening of psychosis  … (versus those who) became too impaired to obtain cannabis.”
As for Large’s most serious claim, that juvenile marijuana use “is a cause of schizophrenia,” most experts on the subject – and most scientific reviews of the matter – disagree.

For example, authors of a 2009 study published in Schizophrenia Research said definitively that increased cannabis use by the public has not been followed by a proportional rise in diagnoses of schizophrenia or psychosis. Investigators at the Keele University Medical School in Britain compared trends in marijuana use and incidences of schizophrenia in the United Kingdom from 1996 to 2005. Researchers reported that the "incidence and prevalence of schizophrenia and psychoses were either stable or declining" during this period, even the use of cannabis among the general population was rising.

"[T]he expected rise in diagnoses of schizophrenia and psychoses did not occur over a 10 year period," they concluded. "This study does not therefore support the specific causal link between cannabis use and incidence of psychotic disorders. ... This concurs with other reports indicating that increases in population cannabis use have not been followed by increases in psychotic incidence."

In April, scientists at the University Hospital of Child and Adolescent Psychiatry in Bern, Switzerland also published clinical trial data indicating that cannabis use plays virtually no role in the early onset of psychosis in younger patients. Researchers assessed the differences in the age of onset of psychosis among 625 patients admitted to the Early Psychosis Prevention and Intervention Centre in Melbourne, Australia. They reported, “Only cannabis use … starting at age 14 was associated with an earlier age at onset at a small effect size.” Overall, the age at onset for patients with first-episode psychosis “was not significantly different” among patients with a history of cannabis use versus non-users.

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