Adolescent Cannabis Use and Outcomes in Young-Adulthood
Post by Leigh Christopher
What's the science?
One common concern with cannabis legalization is the possibility that cannabis use negatively impacts brain development during youth. Many studies have shown links between cannabis use and negative outcomes like mental health problems, cognitive problems, and a reduced ability to obtain education and income later in life. Understanding whether cannabis use is actually the cause of negative outcomes in adolescents is challenging from an experimental perspective. Other genetic and environmental factors might contribute to vulnerability to negative outcomes in response to cannabis use, making it difficult to disentangle which factors are causal. Many studies to date have been limited as they have either 1) examined the impact of cannabis use at one point in time (not over time), or 2) they have not accounted for genetic factors that could influence vulnerability to negative outcomes. This week in PNAS, Schaefer and colleagues used three longitudinal twin studies which fully account for shared genetic and environmental factors, to examine the effects of cannabis use on cognitive, psychiatric, and socioeconomic outcomes in young adults.
How did they do it?
The authors looked for associations between cannabis use in adolescents and negative outcomes in young adulthood using a large sample (3762 participants) that included data from 3 longitudinal twin studies. Analyses conducted in monozygotic (identical) twins account for shared genetic and environmental contributions to the outcome measure of interest since these twins have identical genes and come from the same families. Therefore, twin studies act as a much stronger indicator of causality - a finding that twins who use more cannabis and show more negative outcomes would indicate that the negative outcomes are not due to any confounding genetic or shared environmental vulnerability, but rather are due to the cannabis use itself. Having said that, there are always other potential twin-specific confounders that could differ between a set of twins such as exposure to other drugs. The authors created an adolescent cannabis use index to examine the participants' cannabis use prior to and during adolescence. They examined whether individuals who used more cannabis also experienced more negative cognitive, psychiatric, and socioeconomic outcomes in young adulthood.
What did they find?
Broadly speaking, cannabis use was associated with a number of psychiatric, cognitive, and socioeconomic outcomes such as anxiety, depression, and lower educational attainment. The authors then looked at whether this association still held true after accounting for shared genetic and environmental variability by examining monozygotic twin pairs who differed in terms of their cannabis use. When looking at these twins, the association between cannabis use and cognitive or psychiatric outcomes was no longer significant, suggesting that this association is due to genetic predisposition or other family background factors. The association between cannabis use and socioeconomic outcomes, however, remained significant, indicating that the link between cannabis use and worse socioeconomic outcomes (housing, income, education, occupational status) is not confounded by shared genetic or environmental vulnerability. Since twins may also differ in some environmental factors like exposure to substance abuse in adolescence, the authors performed a follow-up analysis to account for exposure to alcohol and tobacco. They found that the results did not differ when taking these factors into account. Lastly, the authors conducted a follow-up analysis to examine the pathways through which cannabis use might influence socioeconomic outcomes. They found that cannabis use was predictive of worse academic performance, motivation, and problems in school after accounting for shared genetic or environmental vulnerability.
What's the impact?
This study was the first to look at the impact of adolescent cannabis use on multiple adult outcomes using a large, longitudinal sample of twins with repeated assessments of cannabis use administered during the teenage years. These findings suggest that cannabis use does not cause negative cognitive or psychiatric outcomes in adolescents and that these outcomes are more likely driven by shared genetic or environmental vulnerability. However, this study did show that cannabis use is linked to worse socioeconomic outcomes after controlling for genetic factors and that cannabis use likely impacts academic performance leading to worse outcomes in young adulthood.
Schaefer et al. Associations between adolescent cannabis use and young-adult functioning in three longitudinal twin studies. PNAS (2021). Access the original scientific publication here.