The study found links between cannabis use and brain impairment only in the areas of verbal IQ and specific cognitive abilities related to frontal parts of the brain, particularly those that require learning by trial-and-error.
The authors looked at 294 teenagers who were part of the Montreal Longitudinal and Experimental Study, a well-known cohort of 1,037 white French-speaking males from some of the city's poorer neighborhoods. The teenagers completed a variety of cognitive tests at ages 13, 14 and 20 and filled out a questionnaire once a year from ages 13 to 17 and again at 20, between 1991 and 1998.
Nearly half, 43 percent, reported smoking pot at some point during that time, most of them only a few times a year. At 20 years of age, 51 percent said they still used the drug. In general, those who started early already had poor short-term memory and poor working memory (that is, the ability to store information such as a phone number long enough to use it, or follow an instruction shortly after it was given). Conversely, the early users also had good verbal skills and vocabulary; the authors believe drugs are hard to get for young people and that makes them more creative, which suggests the authors know little about drugs outside academic thought.
Besides filling out questionnaires about their use of drugs and alcohol over the previous year, the boys participated in a number of tests to measure their cognitive development. For example, they were given words and numbers to remember and repeat in various configurations, were asked to learn new associations between various images, played a card game to gauge their response to winning or losing money, and, in a test of their vocabulary, had to name objects and describe similarities between words. In general, those who performed poorly in language tests and tests that required learning by trial-and-error, either to make associations between images or to detect a shift in the ratio of gains to losses during the card game, reported smoking pot in their young teens.
The team found smoking cannabis during adolescence was only linked to later difficulties with verbal abilities and cognitive abilities of learning by trial-and-error, and those abilities declined faster in teens who started smoking early than teens who started smoking later. The early adopters also tended to drop out of school sooner, which helped explain the decrease in their verbal abilities.
"Overall, these results suggest that, in addition to academic failure, fundamental life skills necessary for problem-solving and daily adaptation [...] may be affected by early cannabis exposure," the study says.
So delay negative health behavior for as long as possible. This makes sense. As the American Council on Science and Health has long noted, smoking is a pediatric disease. And we can expect marijuana proponents to want to get people started early, just like Big Tobacco did in the 1940s and '50s.
Citation: Natalie Castellanos-Ryan, Jean-Baptiste Pinault, Sophie Parent, Frank Vitaro,"Adolescent cannabis use, change in neurocognitive function, and high-school graduation: A longitudinal study from early adolescence to young adulthood," Development and Psychopathology, DOI: https://doi.org/10.1017/S0954579416001280, 29 December 2016