On Monday, I published a rebuttal of these claims in a commentary published on the website Alternet.org — an excerpt of which appears below.
Debunking the Latest Pathetic Fear Smear Campaign Against Marijuana
[excerpt] [N]umerous (though far less publicized) studies have come to light downplaying the likelihood that cannabis use is a direct cause of psychiatric disorders like schizophrenia. Specifically, a 2009 paper in the journal Schizophrenia Research compared trends in marijuana use and incidences of schizophrenia in the United Kingdom from 1996 to 2005. Authors reported that “incidence and prevalence of schizophrenia and psychoses were either stable or declining” during this period, even though pot use among the general population was rising. 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.”
Similarly, a 2010 review paper published by a pair of British scientists in the journal Addiction reported that clinical evidence indicating that use of he herb may be casually linked to incidences of schizophrenia or other psychological harms is not persuasive. 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.”
More recently, researchers at Harvard University released a study further rebutting this allegation. Writing in 2013 in Schizophrenia Research, investigators compared the family histories of 108 schizophrenia patients and 171 individuals without schizophrenia to assess whether youth cannabis consumption was an independent factor in developing the disorder. Researchers reported that a family history of schizophrenia increased the risk of developing the disease, regardless of whether or not subjects consumed weed as adolescents. They concluded: “The results of the current study, both when analyzed using morbid risk and family frequency calculations, suggest that having an increased familial risk for schizophrenia is the underlying basis for schizophrenia in these samples and not the cannabis use. While cannabis may have an effect on the age of onset of schizophrenia it is unlikely to be the cause of illness.”
In fact, some researchers speculate that specific cannabinoids, such as cannabidiol (CBD), may even be efficacious in treating symptoms of psychosis. According to a review published in the January 2014 issue of the journal Neuropsychopharmacology: “CBD has some potential as an antipsychotic treatment. … Given the high tolerability and superior cost-effectiveness, CBD may prove to be an attractive alternative to current antipsychotic treatment.” Specifically, a 2012 double-blind, randomized placebo-controlled trial assessing the administration of CBD versus the prescription anti-psychotic drug amisulpride in 42 subjects with schizophrenia and acute paranoia concluded that two substances provided similar levels of improvement, but that cannabidiol did so with far fewer adverse side effects.
Case reports in the scientific literature also indicate that some patients turn to cannabis for subjective benefits, though other studies indicate that pot use may exacerbate certain symptoms in patients with psychiatric disorders. Nonetheless, even a recent paper summarizing the “adverse health effects of recreational cannabis use” acknowledges, “It is difficult to decide whether cannabis use has had any effects on psychosis incidence, because even if a relationship were to be causal, cannabis use would produce a very modest increase in incidence.”
You can read my full commentary here.
You can also watch my discussion with Thom Hartmann of The Big Picture (air date: February 23) here.