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Brain researcher cautions against suggestions cannabis causes schizophrenia

September 24, 2015 by Bob Yirka

(Medical Xpress)—Matthew Hill, of the University of Calgary's Hotchkiss Brain Institute has published a Perspective piece in the journal Nature voicing his concerns about other academics and journalists who suggest that cannabis use causes schizophrenia. He points out that there is little evidence of such a connection and suggests that there is actually evidence that cannabis use does not cause the mental disorder.

Cannabis use has been in the news a lot of late, particularly in the U.S. where some states have made it legal to smoke marijuana—the common name for cannabis. But because it is a hot-button topic, some, particularly those opposed to its use, have taken to looking for reasons to stop the movement—and one of those arguments is that it causes schizophrenia. The problem with that, as Hill notes, is that no one has ever proved it to be true. He notes that it is almost certainly not true, noting that the incidence of schizophrenia has not gone up since cannabis use in Europe and the U.S. rose sharply after the 1960's. Also, he notes, countries where a large part of the population use cannabis do not have a higher percentage of schizophrenia patients than countries where few if any people use the drug. Hill does point out that it appears that cannabis use might bring on schizophrenia earlier for those who are going to develop the disorder anyway.
Hill had an article published last year in the Journal of Psychiatry & Neuroscience, called "Clearing the smoke: What do we know about adolescent cannabis use and schizophrenia?" in which he outlined studies that have been conducted to determine if cannabis does indeed cause schizophrenia—the first of which was conducted back in 1987 in Sweden, and did find such an association—army conscripts who used cannabis were found to have a higher incidence of schizophrenia. It was this study that has led to the common belief today that the drug causes the mental illness. But, as Hill notes, what the study actually found was that high doses of tetrahydrocannabinol—the psychoactive component of cannabis, can cause acute psychosis—but only for a short period of time. The researchers back then assumed the condition could be permanent and lead to schizophrenia and thereby made their pronouncement. He notes that no other study has come to the same conclusion.
There have been other studies conducted but not cited by Hill, most prominently one done by a team at Harvard University recently—all have led to the same conclusion—that cannabis use does not cause schizophrenia—but still as Hill notes, the notion continues. He cautions against those who wish to incite and suggests that careful evidence based arguments be used when it comes to making policy.
 Explore further: Cannabis use mimics cognitive weakness that can lead to schizophrenia
More information: Perspective: Be clear about the real risks, Nature 525, S14 (24 September 2015) DOI: 10.1038/525S14a
Journal reference: Nature