Smoking Cannabis and Acquired Impairments in Cognition: Starting Early Seems Like a Really Bad Idea | Philip D. Harvey, PhD, Chief, Division of Psychology ocietal attitudes toward cannabis use are changing rapidly. There has been a wide-ranging movement toward removal of criminal sanctions on cannabis use. This movement has raised some concerns about whether we understand the full implications of cannabis use, particularly in young individuals. Cannabis use is legal for over 200 million residents of the US and 37 million Canadians.The most recent large-scale legalization of cannabis use has occurred in Canada. Effective October, 2018, Canada removed all criminal sanctions on marijuana use. Morin et al. [1] studied changes in cognitive performance associated with cannabis use in 3,826 students in Montreal who were examined annually for a four-year period, starting in seventh grade. First, both alcohol and cannabis use increases annually. Second, the effects of cannabis are considerably greater than those of alcohol. Third, use of cannabis has persistent cognitive effects that worsen with continued use. Although the differences in cognitive performance associated with cannabis use do not wipe out all maturation-associated improvements, they are substantial. Tenth graders who have concurrent use and past year use of cannabis perform like seventh graders who have never used cannabis on measures of inhibitory control. This is a four-year disadvantage.
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As these children grow older, it is very likely that their opportunities for cannabis use will increase. It might be expected that additional cognitive changes would accrue with sustained cannabis use. Meier et al. [2] previously studied cannabis use in an age period similar to the current study (ages 7-13). Participants were reassessed at 18, 21, 26, 32, and 38 years of age. No baseline differences in intelligence between the different use groups were detected; the baseline IQs of those who never used cannabis and those with persistent abuse at age 38 were identical at 99.84 and 99.68 respectively. The cognitive effects of persistent cannabis use were substantial: 6-point declines in total IQs. Informant reports of cognitive limitations were substantial as well. In contrast to recent research on smoking cessation and cognition, reducing cannabis use did not lead to restoration of cognitive functioning. These studies suggests that cannabis-related cognitive changes are detectable and important during middle school and high school years and persistent cannabis with early onset use leads to major cognitive challenges by the late 30’s. There are signs of early-onset and persistent effects of cannabis use on cognition in quite different but very comprehensive studies. If that was the only potential effect of early onset
cannabis use, there would be considerable reason for concern. However, there is a whole other set of risks associated with early-onset cannabis use: increased risk for development of psychosis. Research has suggested that the risk for schizophrenia among cannabis users is about two percent, which is more than double the population base rate. This risk is also increased within individuals with genomic risk, life-stress related factors, and very early onset of use. These studies suggest a need for considerable concern about the impact of the anticipated wider access to cannabis. As the adverse cognitive effects of early-onset cannabis use appear clear, education and prevention efforts will be needed in order to prevent academic decline and other consequences of cannabis use early in life. This piece has been edited for length and clarity. For the full paper, please email psychiatry@med.miami.edu
References [1] Morin JF, Afzali MH, Bourque, J, Stewart SH, SeĚ guin JR, O’Leary-Barrett M, Conrod, PJ. A Population-Based Analysis of the Relationship Between Substance Use and Adolescent Cognitive Development, Am J Psychiatry, [2] Meier MH, Caspi A, Ambler A, et al. Persistent cannabis users show neuropsychological decline from childhood to midlife. Proc Natl Acad Sci USA. 2012;109(40):E2657-64
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