Characterizing poly-substance use and its associations with anxiety and depression among a sample of Canadian high school students

In any given year, one in five Canadians experience mental illness. The majority of mental illnesses originate in adolescence or young adulthood. Many complex factors can contribute to this, including substance use. However, research does not accurately measure how students are using substances. Typically, researchers examine one substance at a time (e.g., alcohol, cannabis, or cigarettes) but 20% of youth use more than one, or poly-substance use. Poly-substance use is the concurrent use of more than one substance (e.g., alcohol, cannabis, or cigarettes) within a specified period (e.g., the past 30 days). To address this research gap, this thesis examined substance use classes among adolescents and how those classes were associated with anxiety and depression, both cross-sectionally and longitudinally. This research will help inform future surveillance, prevention, and treatment efforts for youth substance use and mental health. \n \nRecent evidence indicates that one in five Canadian students in grades 9 to 12 engage in poly-substance use. Common patterns of substance use among adolescents have been identified, including a low use or no use group comprising most adolescents, a single or dual substance use group, a moderate poly-substance use group, and finally a higher poly-substance use group. Longitudinal research indicates that adolescents typically maintain substance use patterns over time but if they make a change, adolescents are more likely to increase rather than to decrease the number of substances they use over time. There is limited study of poly-substance use among Canadian youth and few studies have examined the role of recent increases in e-cigarette use among adolescents in poly-substance use over time. \n \nPoly-substance use has been associated with elevated symptoms of depression and anxiety among adolescents in cross-sectional research. Previous longitudinal research has identified mixed results. Additionally, previous longitudinal studies have not explored sex differences, despite known differences being identified in cross-sectional research and the knowledge that patterns of poly-substance use, anxiety, and depression are known to differ between female and male adolescents. \nThe overall objective of this dissertation was to characterize poly-substance use and examine its associations with anxiety and depression among a sample of Canadian adolescents both cross-sectionally and longitudinally, stratified by sex. Specific objectives were to (1) determine the cross-sectional substance use classes among a sample of Canadian secondary school students and examine their associations with anxiety and depression symptoms, (2) examine the longitudinal bi-directional associations between poly-substance use and anxiety and depression among secondary school students over time and, (3) examine the longitudinal associations between latent classes of substance use and anxiety and depression scores over time among youth who use substances. This thesis made use of data from 3 waves of the COMPASS study (Wave 1: 2017/18, Wave 2: 2018/19, and Wave 3: 2019/20). Students reported their substance use (alcohol, cannabis, cigarettes, and e-cigarettes) and anxiety and depression symptoms at each wave. The first manuscript made use of cross-sectional data (Wave 1: 2017/18) while the second two used longitudinal linked data from all three years (Wave 1: 2017/18, Wave 2: 2018/19, and Wave 3: 2019/20). \n\t \nThe first manuscript examined cross-sectional patterns of substance use among a sample of Canadian secondary school students and their associations with clinically relevant symptoms of anxiety and depression, or both in the 2017/18 school year. Results identified three classes of substance use: poly-use, dual use, and non-use. These results indicate that Canadian secondary school students are engaging in dual and poly-substance use. Those with anxiety, depression, or both had higher odds of being in the poly-use class compared to the non-use class. Symptomology was also associated with belonging to the dual use class except among males with anxiety symptoms only. \n \nAfter identifying a cross-sectional association, the second manuscript used data from students linked over three years to examine the directionality of the association. The objective was to examine the bi-directional associations between (1) number of substances used and anxiety and depression among all students and (2) poly- versus single substance use and anxiety and depression among students who used substances. Among all male students, a uni-directional association was identified where an increase in the number of substances used was associated with reduced anxiety symptoms between Waves 1 and 2. Among students who used substances, uni-directional associations were identified between Waves 2 and 3 where poly-substance use was associated with increased depression among males and increased anxiety among females and males. \n \nFinally, given that the previous manuscript provided evidence that poly-substance use preceded changes in anxiety and depression symptoms among students who used substances, the focus of the final manuscript was on this direction of effect. The third manuscript examined the longitudinal associations between latent classes of substance use over time and anxiety and depression scores among youth who used substances. These analyses identified three classes of substance use over time: occasional alcohol and e-cigarette use, escalating poly-substance use, and consistent poly-substance use. After controlling for relevant covariates, consistent poly-substance use was associated with depression but not anxiety. Additionally, escalating poly-substance use was associated with depression among male students. \n\t \nThis dissertation fills an important gap with respect to our knowledge of poly-substance use and its association with anxiety and depression among adolescents in Canada. The findings highlight the need to consider adolescent poly-substance use and have implications for practice and research. Given that many students reported poly-substance use, surveillance and prevention strategies should consider substance use patterns, including dual and poly-substance use. As this research identified substance use as a potentially modifiable behaviour associated with adolescent mental health, it may also be useful to consider a mental health component in substance use programming. Future research examining this association should additionally consider simultaneous poly-substance use, clinical measures of anxiety and depression, and capture reasons for substance use to further strengthen and understand the association between poly-substance use and anxiety and depression among adolescents.

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