Smoking Cannabis and Acquired Impairments in Cognition: Starting Early Seems Like a Really Bad Idea.

Societal attitudes toward cannabis use are changing rapidly. In the past decade, there has been awide-ranging movement toward removal of criminal sanctions on cannabis use, and in some localities cannabis use is permitted in a regulated manner, similar to alcohol. This movement has raised some concerns about whether we understand the full implications of cannabis use, particularly in younger individuals. Although cannabis is clearly associated with fewer fatal consequences than alcohol use, it has long been understood that cannabis use can be associated with persistent and substantial cognitive changes (1) and possibly alterations in motivational processes (2). The most recent large-scale legalization of cannabis use has occurred in Canada. Effective October 17, 2018, Canada removed all criminal sanctions on marijuana use, in a move characterized by Prime Minister Justin Trudeau as improving the safety of the population by removing the influence of organized crime on the distribution of marijuana (3). With cannabis use legal in nine U.S. states and the District of Columbia, this means that over 200 million residents of the United States and 37 million Canadians have legal access to recreational cannabis. In that context, the article by Morin et al. in this issue (4) is particularly timely. The authors studied the longitudinal changes in cognitive performance associated with cannabis use in 3,826 students in Montreal who were examined annually over a 4-year period. The strengths of the study include annual assessments with cognitive tests, a starting time in seventh grade,which captures a critical period for both brain development and substance use, and a comparison of cannabis effects with the effects of alcohol. Cognitive performance was examined with performance-based assessments that have known validity and standardization for this age group. Analyses compare both the between-subject differences associated with types and levels of substance use and, more interestingly, the within-subject effects of substance use on cognitive performance.The latter analysis can actually pinpoint year-by-year within-individual changes associated with substance use. The authors discovered several interesting things. First, all elements of both alcohol and cannabis use increase annually across the various levels of frequency of use, with the exception of daily alcohol use. Second, the effects of cannabis are considerably greater than those of alcohol. Third, there appear to be both concurrent and lagged effects of cannabis use on several cognitive domains. Thus, use of cannabis has persistent effects that are also exacerbated by continued use. In this study, the authors could not estimate actual dosing of cannabis in the same way that they could for alcohol. Also, the proportion of alcohol users who would be considered possible binge drinkers was 20% or less, so drinking was more common than cannabis use, but the number of participants with very high doses of alcohol was limited. This is probably due to the young age of the sample and the relative difficulty of gaining access to high doses of alcohol at that age. Although the effects on cognitive performance associated with cannabis use do not block maturation-associated improvements in performance on the tasks across all domains, they are substantial in some areas. For example, in relation to inhibitory control, tenth graders who have concurrent use and past-year use of cannabis perform like seventh graders who have never used cannabis. This is a 4-year disadvantage. Performance on measures of inhibitory control also shows a between-subject effect, in that all of the cannabis user groups make about 25% more errors at the grade 7 baseline assessment than those participants who never showed signs of developing cannabis use over the 4-year period. What is concerning about these data is that this deficit in inhibitory control shows an early, persistent, and dosedependent association with cannabis use in a very early age group with relatively infrequent use. As these children grow older, it is very likely that their opportunities for cannabis use will increase with naturally occurring decreases in parental supervision. Interacting with these anticipated reductions in supervision is the widely expected increased access to cannabis that will occur with legalization. Extrapolating forward, it might be expected that additional cognitive changes would accrue with sustained cannabis use. Meier et al. (1) previously presented results from theDunedin study of adult consequences of adolescent-onset cannabis use. There are several parallels to the present study. In the Dunedin study, onset of cannabis use was examined in an age period similar to that of the Morin et al. study (ages 7–13). Participants were reassessed at ages 18, 21, 26, 32, and Extrapolating forward, it might be expected that additional cognitive changes would accrue with sustained cannabis use.