Supposed Marijuana And Schizophrenia Link “Overstated”

[Editor's note: This post is excerpted from this week's forthcoming NORML weekly media advisory. To have NORML's media advisories delivered straight to your in-box, sign up for NORML's free e-zine here.]

Clinical evidence indicating that marijuana use may be casually linked to incidences of schizophrenia or other psychological harms is not compelling, according to a scientific review published online by the journal Addiction.

Investigators at the University of Bristol, Department of Social Medicine assessed the potential health risks of cannabis, particularly whether use of the drug may be causally linked with mental illness.

Authors wrote: “We continue to take the view that the evidence that cannabis use causes schizophrenia is neither very new, nor by normal criteria, particularly compelling. … For example, our recent modeling suggests that we would need to prevent between 3000 and 5000 cases of heavy cannabis use among young men and women to prevent one case of schizophrenia, and that four or five times more young people would need to avoid light cannabis use to prevent a single schizophrenia case. … We conclude that the strongest evidence of a possible causal relation between cannabis use and schizophrenia emerged more than 20 years ago and that the strength of more recent evidence may have been overstated.”

In 2007, an analysis in the British medical journal The Lancet estimated that experimenting with marijuana could increase one’s risk of developing a psychotic illness later in life by some 40 percent. Following this report, Parliament in 2008 voted to reclassify marijuana as a Class B substance, making its possession punishable by up to five years in prison.

University of Bristol researchers also criticized Parliament’s reclassification of the drug, which took effect earlier this year. They concluded: “The only important possible benefit of prohibition is prevention of cannabis use. There is little or no evidence that it effectively achieves this benefit. Patterns of cannabis use in the population appear to be independent of the policy surrounding use, and criminalizing individual cannabis users does not appear to modify their use in a healthy way.”

Overall, investigators determined that marijuana’s most significant health risk was its association and reinforcement with tobacco smoking.

Study Debunks Claim That Pot Smoking Causes Mental Illness

Much has made — by the mainstream media and others — of the claim that cannabis use causes certain types of mental illness, specifically schizophrenia and psychosis.

Most notably perhaps, a team of researchers writing in the July 28, 2007 edition of the prestigious scientific journal The Lancet, boldly proclaimed that smoking cannabis could boost one’s risk of a psychotic episode by 40 percent or more.

Naturally, this alarmist rhetoric received wall-to-wall coverage by the mainstream press. Even more troubling, the supposed ‘pot-and-schizophrenia’ link was one of the primary reasons cited by British PM Gordon Brown, ex-Home Secretary Jacqui Smith and others as the impetus for reclassifying cannabis (from a verbal warning to a criminal offense punishable by up to five years in jail) in the United Kingdom.

Of course, there was a fatal flaw with The Lancet‘s argument — one that, oddly enough, every single MSM outlet failed to mention. Empirical data did not support the investigators’ hypothesis that smoking marijuana was associated with increased rates of schizophrenia or other mental illnesses among the general public — a fact that even the authors begrudgingly admitted when they declared, “Projected trends for schizophrenia incidence have not paralleled trends in cannabis use over time.”

Which brings us to 2009.

Two years after The Lancet‘s dire predictions, a team of researchers at the Keele University Medical School have once and for all put the ‘pot-and-mental illness’ claims to the test. Writing in a forthcoming edition of the scientific journal Schizophrenia Research, they compare long-term trends in marijuana use and incidences of schizophrenia and/or psychoses in the United Kingdom. And what do they find?

“[T]he expected rise in diagnoses of schizophrenia and psychoses did not occur over a 10 year period. 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.”

Should we expect an apology — or even better, a change in policy — from the Gordon Brown regime any time soon? Or at the very least, will some sort of ‘correction’ be forthcoming from the mainstream news media?

I wouldn’t hold my breath.