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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.

 Prohibitionists have a long history of exploiting tragedy to further their own drug war agenda. Case in point: Members of Congress in the 1980s seized upon the overdose of basketball star Len Bias to enact sweeping legislative changes establishing mandatory minimum sentencing in drug crimes, random workplace drug testing for public employees, and the creation of the Drug Czar’s office.

So it was hardly surprising to see anti-drug zealots return to this tried-and-true playbook in the days immediately following the shooting this past January of Arizona Congresswoman Gabrielle Giffords and 18 civilians. Only hours after alleged shooter Jared Lee Loughner was taken into custody, pundits on the political far right opined that the 22-year-old former pot smoker had been driven mad by weed.

For example, less than 24-hours after the shooting former George W. Bush speech-writer David Frum p osed the question, “Did pot trigger the Giffords shooting?” to which the longtime conservative commentator answered, “Increasingly, experts seem to be saying ‘yes.’”

Frum’s accusation appeared to gain a modicum of respectability one month later when the mainstream media highlighted a report in  The Archives of General Psychiatry  that purported to have linked marijuana use with psychosis.

“It is increasingly clear that marijuana is a cause of schizophrenia,” the study’s lead researcher, Matthew Large of Prince of Wales Hospital in New South Wales, Australia, told the online publication Web MD in February. (In a separate interview he said he was “horrified” by suggestions that the plant should be legalized and regulated.) Large further insisted, “[T]he schizophrenia caused by cannabis starts earlier than schizophrenia with other causes.”

Or not.

In truth, the supposed new ‘study’ contained no new findings at all. Rather, Large and his team simply reviewed previously published research – much of it decades old.

“There are no new data. I want to emphasize that. This is a meta-analysis, which means it (reviews) the studies that were already out there,” SUNY Albany psychology professor Mitch Earleywine, author of the book  Understanding Marijuana: A New Look at the Scientific Evidence , explained on the NORML Audio Stash days after the report’s release. “What you’re not hearing in the media is that in fact, this (reported association) is probably early-onset folks self-medicating (with cannabis).”

There are several published reports to back up Earleywine’s suspicion. For instance, a 2005 study of 1,500 subjects that appeared in the scientific journal  Addiction reported that the development of “psychotic symptoms in those who had never used cannabis before the onset of (such) symptoms … predicted future cannabis use.”

Other studies reinforcing Earleywine’s ‘self-medication’ theory include a 2008 study published in the International Journal of Mental Health Nursing  which found that schizophrenics typically report using cannabis to reduce anxiety and “improve their mental state.” Marijuana use has also been associated with clinically objective benefits in some schizophrenics. Recently, a 2010 report in the journal Schizophrenia Research  found that schizophrenic patients with a history of cannabis use demonstrate higher levels of cognitive performance compared to nonusers. Researchers in that study concluded, “The results of the present analysis suggest that (cannabis use) in patients with SZ (schizophrenia) is associated with better performance on measures of processing speed and verbal skills. These data are consistent with prior reports indicating that SZ patients with a history of (cannabis use) have less severe cognitive deficits than SZ patients without comorbid (cannabis use).”

A 2011 meta-analysis published online by the journal  Schizophrenia Research  also affirmed that schizophrenics with a history of cannabis use demonstrate “superior neurocognitive performance” compared to non-users. Investigators at the University of Toronto, Institute of Medical Sciences reviewed eight separate studies assessing the impact of marijuana consumption on cognition, executive function, learning, and working memory in schizophrenic subjects. Researchers determined that the results of each of the performance measurements suggested “superior cognitive functioning in cannabis-using patients as compared to non-using patients.”

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