Cannabis use during adolescence is associated with altered brain development, according to a new study published in JAMA Psychiatry. The new research represents the largest longitudinal neuroimaging study of cannabis use to date.
Although some studies have found evidence that adolescents who use cannabis tend to have reduced cortical thickness in frontal brain regions, the cross-sectional nature of past research has left it unclear whether these differences in brain structure are a consequence of the drug itself.
“Despite increasing trends in cannabis legalization, there have been surprisingly few longitudinal imaging studies on this topic. Most imaging studies of cannabis use have been relatively small in sample size and cross-sectional in nature,” said study author Matthew D. Albaugh, a licensed clinical psychologist and assistant professor of psychiatry at the University of Vermont.
To overcome these limitations, Albaugh and his colleagues analyzed magnetic resonance imaging data from 799 participants in the IMAGEN (Imaging Genetics for Mental Disorders) study, which has followed European adolescents from the age of 14 onwards. The participants underwent brain scans at the start of the study and during a 5-year follow-up.
The researchers found that adolescents who reported moderate-to-heavy cannabis use tended to have reduced thickness in left and right prefrontal cortices, an area of the brain involved in planning, decision making, working memory and learning. There was a dose-dependent relationship between cannabis use and cortical thickness. That is, those that used more cannabis had more cortical thinning in these brain regions. This relationship held even after accounting for preexisting differences in brain structure.
The altered neurodevelopment also appeared to have some behavioral consequences. The researchers found that cannabis-related cortical thinning was associated with higher levels of impulsiveness.
“Taken together, these results provide strong circumstantial evidence that cannabis use during adolescence is associated with altered neurodevelopment — particularly in cerebral cortices rich in cannabinoid 1 receptors and undergoing the greatest age-related change in middle to late adolescence. Our findings dovetail with animal research suggesting that adolescence may be a particularly vulnerable period with regard to the effect of tetrahydrocannabinol (THC) on brain structure and function,” Albaugh told PsyPost.
“The observed cannabis-related brain effects in this study may serve an important role in linking adolescent cannabis use with various negative outcomes, including adult psychiatric symptoms.”
The researchers controlled for factors such as alcohol consumption, baseline cortical thickness, sex, socioeconomic status, and IQ scores. But it is still possible that a third factor explains the apparent association between cortical thinning and cannabis use.
“As is the case with all human research on this topic, our study was observational in nature. As a result, we cannot rule out the possibility that preexisting cognitive and/or behavioral differences were associated with the observed neurodevelopmental trajectories and that cannabis use was not causally related to cerebral cortical development,” Albaugh explained.
“Although such an alternative explanation is possible, several observations from the present study are worth reiterating. First, there was a dose-dependent association at 5-year follow-up between lifetime cannabis use and cortical thickness. Second, there were no significant associations between baseline cortical thickness and lifetime cannabis use at 5-year follow-up. Third, the spatial pattern of cannabis-related thinning was significantly associated with a PET-derived map of CB1 receptor availability.”
The researchers say that their findings highlight the importance of conducting longitudinal studies to examine the consequences of cannabis use.
“We are currently examining the degree to which cannabis use in early adulthood (from 19 to 23 years of age) affects brain maturation — and, specifically, if the brain effects we observed in the present study are specific to adolescent cannabis use,” Albaugh said. “There is compelling evidence from animal studies that adolescent cannabis exposure produces qualitatively different brain changes when compared to cannabis exposure during adulthood.”
The study, “Association of Cannabis Use During Adolescence With Neurodevelopment“, was published June 16, 2021.