Adolescent cannabis use, change in neurocognitive function, and high-school graduation

Castellanos-Ryan N, Pingault JB, Parent S, Vitaro F, Tremblay RE, Séguin JR. Adolescent cannabis use, change in neurocognitive function, and high-school graduation: A longitudinal study from early adolescence to young adulthood. Dev Psychopathol. 2016 Dec 29:1-14.

Abstract

The main objective of this prospective longitudinal study was to investigate bidirectional associations between adolescent cannabis use (CU) and neurocognitive performance in a community sample of 294 young men from ages 13 to 20 years. The results showed that in early adolescence, and prior to initiation to CU, poor short-term and working memory, but high verbal IQ, were associated with earlier age of onset of CU. In turn, age of CU onset and CU frequency across adolescence were associated with (a) specific neurocognitive decline in verbal IQ and executive function tasks tapping trial and error learning and reward processing by early adulthood and (b) lower rates of high-school graduation. The association between CU onset and change in neurocognitive function, however, was found to be accounted for by CU frequency. Whereas the link between CU frequency across adolescence and change in verbal IQ was explained (mediated) by high school graduation, the link between CU frequency and tasks tapping trial and error learning were independent from high school graduation, concurrent cannabis and other substance use, adolescent alcohol use, and externalizing behaviors. Findings support prevention efforts aimed at delaying onset and reducing frequency of CU.

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Using a community sample of young men ages 13 to 20, the researchers looked a variety of cognitive measurements to undercover potential differences between those who used marijuana and those who did not.

From 1991 and 1998, the participants completed cognitive tests at ages 13, 14, and 20. They also answered questionnaires at ages 13, 14, 16, 17, and 20. Of the 294 participants, 43% had used pot at some point during the study period—most reported that it was only a few times a year. Results showed that 51% of participants had used marijuana by age 20.

The young men who started using marijuana early typically already had poor short-term memory and working memory, such as remembering a phone number after it was given. They were more likely to drop out of high school than non-smokers. However, these early pot users typically had good verbal skills and vocabulary. Lead author, Natalie Castellanos-Ryan, PhD, hypothesized that these results could have developed as the students found ways to get possession of drugs.

Smoking at the age of 17 or older appeared to have less risk. “We found that adolescents who started using cannabis at 17 or older performed equally well as adolescents who did not use cannabis,” explained Castellanos-Ryan, an assistant professor at UdeM’s [Université de Montréal]School of Psychoeducation.

Difficulties in both verbal and cognitive abilities emerged later in life for those who smoked during adolescence. These problems occurred sooner in those who started smoking pot earlier, as described in the journal article in Development and Psychopathology. 

“The results of this study suggest that the effects of cannabis use on verbal intelligence are explained not by neurotoxic effects on the brain, but rather by a possible social mechanism: Adolescents who use cannabis are less likely to attend school and graduate, which may then have an impact on the opportunities to further develop verbal intelligence,” Castellanos-Ryan continued. 

Marijuana can hurt teenagers cognitively, and is linked to increased dropout rates; so what’s the solution? 

Castellanos-Ryan said that when talking to adolescents about pot, it’s important to avoid exaggerating negative effects. Instead, she says to encourage a delay in smoking. “We can’t tell children, ‘If you smoke cannabis you’re going to damage your brain massively and ruin your life,’” Castellanos-Ryan explained. “We have to be realistic and say, ‘We are finding evidence that there are some negative effects related to cannabis use, especially if you start early, and so, if you can hold off as long as you can – at least until you’re 17 – then it’s less likely there’ll be an impact on your brain.” 

Next up in this research is seeing if the results are replicated in other population samples. Castellanos-Ryan also wants to examine cannabis’ influence on drug abuse and other issues later in life. 

The study, “Adolescent cannabis use, change in neurocognitive function, and high-school graduation: A longitudinal study from early adolescence to young adulthood,” was published in Development and Psychopathology. The news release was provided by UdeM.

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