New Study Links Marijuana to Schizophrenia


As two states have voted to legalize recreational marijuana, and several others will vote on legalization in upcoming elections, the federal government has been evaluating the information about the potential risks related to marijuana use. On June 5, 2014, members of the National Institute on Drug Abuse (NIDA) published the results of their evaluation in the New England Journal of Medicine.

One of the major findings that the NIDA representatives reported is a connection between heavy or long-term marijuana use and a greater risk of schizophrenia among people who have a genetic predisposition for psychosis disorders.

The Complex Causes of Psychosis Disorders

Genetic predisposition has been known for some time to play a role in the emergence of psychosis disorders like schizophrenia, and our understanding of the genetic role in these illnesses has recently been strengthened by the identification of 80 new genes that are linked to schizophrenia. But research has also shown that genetic predisposition does not fully explain who develops schizophrenia. Experts believe that both genetic factors and various behavioral and environmental factors combine to trigger this serious illness.

Previous Study Also Shows Schizophrenia/Cannabis Link

The NIDA evaluation is not the first review of marijuana research to conclude that there may be a connection between long-term or heavy cannabis use and higher rates of schizophrenia. A May 2014 review published in the journal Frontiers in Psychiatry concluded: “The evidence indicates that cannabis may be a component cause in the emergence of psychosis.” The results also suggested that 8 percent to 14 percent of schizophrenia cases may be the result of marijuana use.

Reviews Fail to Demonstrate Plausibility or Causality

While the results of these comprehensive reviews may be cause for concern, and are certainly cause to pursue further research, they are by no means conclusive. The authors of both reviews acknowledge that the information they gathered fails in several ways to draw a definite cause-and-effect link between marijuana use and the development of psychosis disorders.

One major limitation of the information is the lack of a biological explanation for how marijuana use might be causing schizophrenia. This is known as a “biologically plausible mechanism” through which cannabis consumption could be affecting the brain and increasing the risk of psychosis.

Another major question mark hovering over the potential link between drug and illness is the discrepancy between increased marijuana use and stable schizophrenia figures. Over the last 40 years, marijuana use in the United States has increased dramatically. If using marijuana proved to be an important component of schizophrenia, researchers would expect to see a corresponding increase in schizophrenia cases over this period. In fact, the rate of this psychosis disorder in the general population has not increased significantly.

For long-term or heavy marijuana use to be confirmed as one of the causes of schizophrenia, experimental evidence is also necessary. Until these missing links are added or explained, it remains possible that there is another explanation for the association between marijuana and schizophrenia (apart from a cause-and-effect association).

Concerns Over Lack of Data

The fact that research surrounding a possible link between schizophrenia and marijuana is in its early stages highlights some of the concerns that critics of marijuana legalization have. Marijuana research has been relatively limited over the last few decades, and some of those who oppose widespread recreational legalization fear that the potential long-term health risks of marijuana use are still relatively unexplored. They believe that the limited information about marijuana use inaccurately bolsters marijuana’s widespread public reputation as a fairly harmless drug.

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