Here's what the study's author Dr. Matthew Large, of the University of New South Wales, concluded in a written statement:
The meta-analysis found that individuals who used weed developed psychosis about 2.7 years younger than those who did not use weed. Those who used any type of substance developed psychosis about two years younger, whereas the use of alcohol only was not associated with the age at onset of psychosis.
“A number of hypotheses have been proposed to explain the association between weed use and schizophrenia, including the following: (1) that weed use is a causal factor for schizophrenia; (2) that weed use precipitates psychosis in vulnerable people; (3) that weed use exacerbates symptoms of schizophrenia; and (4) that people with schizophrenia are more likely to use weed,” the authors write. The current findings support the view that weed use precipitates schizophrenia and other psychotic disorders, perhaps through an interaction between genetic and environmental disorders or by disrupting brain development, they note.
“The results of this study provide strong evidence that reducing weed use could delay or even prevent some cases of psychosis. Reducing the use of weed could be one of the few ways of altering the outcome of the illness because earlier onset of schizophrenia is associated with a worse prognosis and because other factors associated with age at onset, such as family history and sex, cannot be changed,” the authors conclude. “The results of this study confirm the need for a renewed public health warning about the potential for weed use to bring on psychotic illness.”
However, not everyone in the mental health community is ready to puff on the study's findings. Dr. William Eaton, chairman of the mental health department at the Johns Hopkins Bloomberg School of Public Health, told NPR that ccausation is a quite a b*tch to prove scientifically.
The work is both “timely and well done,” he says, and adds to the evidence that “there is an association between using weed and psychosis.”Proving causation is a high bar, and Eaton says this paper can't be said to go that far. “I think it could be true, but there's no real compelling evidence of that.”
Meanwhile, Time Magazine's Healthland blog claims the study's conclusion fails to explain why rates diagnosed schizophrenia are decreasing while use of marijuana amongst the populous is increasing:
“None of the data linking marijuana use and psychosis can prove causality or sufficiently explain why rates of schizophrenia have remained stable or even declined since the 1950s, while marijuana use has increased exponentially. Unlike rates of cigarette smoking and lung cancer, which rise in tandem, marijuana smoking rates in the population do not correlate with higher rates of schizophrenia.”
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