Cannabis use in adolescence : studies of trends, correlates and adverse mental health consequences

Background: Cannabis is currently the most commonly used illicit psychoactive substance. Cannabis use is typically initiated during adolescence and is more common among young people than among adults. Cannabis use is associated with negative health and social consequences and early initiation increases the risk of negative outcomes. Cannabis may be particularly harmful to individuals whose brain is not yet fully developed.

Overall aims: The overall aim was to improve our understanding of various aspects of the epidemiology of cannabis use among adolescents. The four studies within the thesis highlight the following main research questions regarding adolescents: (I) Are trends in cannabis use related to trends in drinking? (II) Is socioeconomic status (SES) related to cannabis use? (III) Do pupils in introductory programs (IPs) have an increased risk of cannabis use compared with pupils in national programs? (IV) Is there an association between cannabis use and mental distress?

Data and methods: Data for three of the four studies came from a nationally representative, self-reported, and anonymous school survey conducted annually by the Swedish Council for Information on Alcohol and Other Drugs. The survey is cross-sectional and the sample consists of Swedish 9th grade pupils (15–16 years of age) and pupils in year 2 in upper secondary school (17–18 years of age). To assess whether trends in cannabis use were related to trends in drinking, autoregressive integrated moving average (ARIMA) time-series analysis was used (Study I). To assess the association between self-reported cannabis use and socioeconomic status (SES) and academic orientation, respectively, multi-level mixed-effects Poisson regression with robust standard errors was used. Because the pupils were clustered in school classes, the assumption of independence between observations was not met. Hence, data were analyzed with a random intercept for the school/school class level (Studies II and III).

In the fourth study, data stemmed from the Young in Norway Longitudinal Study. The study covered a broad range of topics and followed a cohort of young people prospectively over 13 years. The cohort was assessed in 1992, 1994, 1999, and 2005. Only those who participated in all four waves and who were aged 11–18 years in 1992 were included in the analysis (n = 1,988). The within-person association between increased cannabis use and mental distress was assessed using fixed-effects modeling.

Results: Alcohol and cannabis consumption functioned neither as complements nor as substitutes in the general population of Swedish 15–16-year-olds. Among cannabis users, there was a positive association between consumption of alcohol and cannabis use. However, this association had become weaker over time, suggesting that alcohol and cannabis used to be complements, but no longer are (Study I). Childhood SES was associated with cannabis use. Those who had at least one parent with higher education were at higher risk of lifetime cannabis use, but had a lower risk of frequent (51+ times) cannabis use. Truancy, risk assessment of cannabis, and parental attitude attenuated the latter association (Study II).

Academic orientation was associated with frequent (21+ times) cannabis use. Pupils from IPs and vocational programs had an increased risk of having used cannabis 21+ times compared with pupils in higher education preparatory programs. However, the association was attenuated and no longer significant when SES, truancy, school dissatisfaction, and early onset of substance use were adjusted for. Moreover, including pupils from IPs when calculating the prevalence of cannabis use did not alter the national estimate of cannabis use among Swedish adolescents aged 17–18 years (Study III).

Changes in cannabis use during adolescence and young adulthood were associated with increased risk of anxiety, symptoms of depression, and suicidal ideation among males. In females, increased cannabis use was associated with an increased risk of anxiety and suicidal ideation (Study IV).

Conclusion: Cannabis and alcohol act neither as complements nor as substitutes among Swedish adolescents. Hence, prevention strategies aimed at either substance are unlikely to affect use of the other. Moreover, the findings strengthen the assumption that cannabis use in adolescence and young adulthood is associated with adverse mental health consequences. The thesis also shows SES differences in cannabis use, related to both parental education level and academic orientation. Adolescents whose parents lack higher education and those who attend a vocational program or IP at upper secondary school have an increased risk of using cannabis more frequently. Overall, the results indicate that young people's cannabis use is an important public health issue and that it is important to try to prevent cannabis use. The thesis also highlights that there are groups that are particularly vulnerable where targeted efforts may be needed to reduce the use of cannabis.

List of scientific papers

I. Gripe, I., Danielsson, A. K., Ramstedt, M. (2018) Are changes in drinking related to changes in cannabis use among Swedish adolescents? A timeseries analysis for the period 1989-2016. Addiction. 2018;Apr 21.
https://doi.org/10.1111/add.14244

II. Gripe, I., Danielsson, A. K., Karlsson, P., Thor, S., Ramstedt, M. (2021) Are the well-off youth in Sweden more likely to use cannabis? Drug and Alcohol Review. 40 (1), 126-134.
https://doi.org/10.1111/dar.13139

III. Gripe, I., Ramstedt, M., Karlsson, P., Thor, S. Academic orientation and cannabis use – Findings from a population-based study of Swedish adolescents in upper secondary school. [Manuscript.]

IV. Gripe, I., Pape, H., Norström, T. Association between cannabis use and mental distress in young people: a longitudinal study. [Submitted.]