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Revisiting Research




One of the biggest arguments from skeptics’ points of view on cannabis legalization is its effect on children, both young and teen-aged. This opinion is born from research papers that warn that teens who smoke cannabis will have lower IQs, experience stunted brain development, lose cognitive abilities, develop psychosis and undergo depression and other mental illnesses. However, for many of these negative studies, there is at least one alternative study that has debunked the reported risks.

An all-time favorite study used by anti-cannabis supporters comes from a 2012 Duke University analysis of 1,000 New Zealand residents between the ages of 13 and 38. It concluded that teenagers who smoked cannabis at least four times a week sustained an average IQ loss of eight points. However, in 2014, a much larger study involving 2,612 children, and conducted by the University College of London found “no relationship between cannabis use and lower IQ at age 15.” The authors further commented that “alcohol use was found to be strongly associated with IQ decline.”

Even scarier was the report published by researchers in 2014 at the Center for Brain Health at The University of Texas in the Proceedings of the National Academy of Sciences describing abnormalities in brain function and structure of long-term cannabis consumers. Reporting that chronic cannabis consumers have smaller brain volume in the orbitofrontal region, the authors went on to claim “the structural connectivity or ‘wiring’ of the brain starts degrading with prolonged marijuana use.” But yet again, a different study, published in the Journal of Neuroscience by researchers at the University of Colorado and the University of Louisville in 2015, cited serious inconsistencies in the 2014 University of Texas report, concluding the negative studies were flawed because they “did not adequately exclude the effects of confounding variables” pointing out that “alcohol use, or other factors, may explain some of the contradictory findings.”

Appealing to authorities to back up their scare stories, cannabis legalization opponents still quote Office of National Drug Control Policy Director John Walter’s following statement at a press conference from over 10 years ago. “New research being conducted here and abroad illustrates that marijuana use, particularly during the teen years, can lead to depression, thoughts of suicide and schizophrenia.” Paul Casadonte, MD, director of substance abuse treatment programs at New York’s Department of Veterans Affairs Medical Center disagrees with Walters’ statement in a 2012 study published in Psychological Medicine, which concluded that “The current data do[es] not indicate that low to moderate lifetime cannabis use is a major contributor to psychosis or poor social and role functioning in clinical high-risk youth with attenuated positive symptoms of psychosis.”

“Associations between cannabis use and cognitive functioning in cross-sectional studies of adolescents and young adults are small and may be of questionable clinical importance for most individuals.”


The most ominous study of all is a 2016 report funded by the National Institute on Drug Abuse (NIDA) whose lead author, Nora Volkow, MD, is the Director of NIDA. The report begins with the following questionable assertion. “It is well established that cannabis use causes acute impairment in the ability of the brain to hold information (i.e., cognitive capacity).” The report published in the Journal of the American Medical Association (JAMA) goes on to claim that “the amotivational effects of chronic cannabis use were linked to impairments in learning and sustained attention” and that “cannabis use causes acute impairment of learning and memory, attention, and working memory.”

Once again, an exhaustive report published in the March 2018 JAMA by researchers with no NIDA affiliation reviewed data from 69 separate studies involving 8,727 subjects and found the JAMA report to be inaccurate or exaggerated. The authors concluded, “Associations between cannabis use and cognitive functioning in cross-sectional studies of adolescents and young adults are small and may be of questionable clinical importance for most individuals.” Regardless of a subject’s age of initiation, the report found no significant long-term deficits in memory, attention, or other aspects of cognitive functioning due to cannabis use.

Most of the studies alleging detrimental results from cannabis consumption by adolescents fail to account for the concomitant use of alcohol, which has been found in multiple studies to cause the problems in adolescents that opponents ascribe to cannabis.

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