Prediction of use of cannabis by young adults in an Australian birth cohort

Cannabis is the most commonly used and abused illicit drug in developed countries, including Australia. Use of cannabis usually begins in adolescence and early adulthood, with the peak age of onset between 16 and 18 years. While cannabis use is widespread, relatively little is known about the family and environmental circumstances that may be implicated in the decision to use cannabis. Such information may provide a basis for developing causal hypotheses to understand better why some people use or become dependent on cannabis. The literature formally distinguishes between use of cannabis, abuse of cannabis and dependence on cannabis, grouping the latter two patterns together under the term ‘cannabis use disorders’. The investigations included in this thesis examine the relationships of selected family and individual factors with cannabis use and cannabis use disorders (CUD) in early adulthood. The specific objectives were to examine: 1) whether maternal marital status, changes in maternal marital status, and quality of the maternal marital relationship are associated with young adults’ cannabis use and use disorders; 2) whether a history of paternal arrest or incarceration predicts young adults’ use of cannabis; 3) whether maternal smoking and alcohol consumption are associated with the child’s use of cannabis in early adulthood; 4) whether there is an association between mental health and behavioural problems in children and their later cannabis use and use disorders; 5) whether childhood sexual abuse predicts young adults’ cannabis use; 6) whether there is a relationship between childhood school performance and cannabis use in early adulthood; and 7) whether these associations are independent of other factors that may confound the link between the cited exposures. The data were taken from the Mater-University of Queensland Study of Pregnancy (MUSP), a longitudinal study of mothers and children which began in Brisbane, Australia, in 1981. Some 7223 mothers and their live singleton children constituted the birth cohort. Data were initially collected when pregnant women presented for their fist antenatal visit (at an average of 18 weeks’ gestation). Mothers were re-interviewed 3-5 days after the birth of their child, and again when the child was 6 months, 5, 14 and 21 years of age. Children were also interviewed at the 14- and 21-year follow-ups. Written informed consent from the mother was obtained at all data collection phases and from the young adult at the 21-year follow-up of the study. Ethics committees at the Mater Hospital and the University of Queensland approved each phase of the study. Young adults’ cannabis use and CUD were assessed at the 21-year follow-up via a self-report questionnaire and the CIDI-Auto, an internationally standardised psychiatric assessment. The independent variables on interest — maternal marital status, changes in maternal marital status, quality of marital relationships, history of paternal arrest and incarceration, maternal smoking and alcohol use, child mental health and problem behaviours, childhood sexual abuse, and childhood school performance — were assessed between child age 5 and 21 years. Potential confounding and mediating factors were prospectively measured between the child’s birth and the 14-year follow-up. Binomial and multinomial logistic regression were used to estimate the risk of relevant outcomes for each independent variable, with associations summarised as odds ratios (ORs) and 95% confidence intervals (CIs). The results of adjusted analyses showed that children whose mothers frequently (three or more times) changed their marital status during the child’s development were more likely to report use of cannabis by early adulthood. In addition, adolescents who grew up in step-father families were more likely to report CUD in early adulthood, and a moderate association was found for those children who lived in a family with a high level of marital disagreement. The present analyses also indicated that frequent use of cannabis at age 21 was more likely among young adults with a history of maternal partner arrest. However, there was no significant association between use of cannabis at age 21 and either maternal partner incarceration or single motherhood. Further, maternal smoking at 14 years was associated with more frequent use of cannabis in offspring at 21 years, regardless of maternal smoking at 5 years. In regard to individual factors, attention problems, delinquency, and aggression were associated with both CUD and multiple substance use disorders in early adulthood, with delinquent behaviour being strongest predictor for CUD. In the present investigations, symptoms of anxiety and depression at 14 years were not associated with young adults’ cannabis use or CUD. The data also showed that age of onset and persistence of problem behaviour in children were associated with young adults’ CUD. Childhood sexual abuse was associated with use of cannabis in early adulthood, and especially with frequent use of cannabis. Finally, children who had lower school performance at 14 years were more likely to report use of cannabis in early adulthood. All of these patterns persisted after statistical control for a number of potential confounders and, where relevant, in sensitivity analyses that, for example, omitted individuals who had already begun to use cannabis by the time that the 14-year follow-up was conducted. In addition, both follow-up of the cohort and participation in the CIDI-Auto in early adulthood were incomplete, with demonstrable potential for selection bias and additional confounding related to it. However, the principal results described above were apparent even when selective attrition within the cohort was taken into account. What, then, has been learned about why some people use cannabis and others do not? Young adults who report use of cannabis or who have evidence of cannabis abuse or dependence at this stage by 21 years differ in a number of ways from those who have never used cannabis. If the demonstrated relationships with the independent factors examined in this study are causal, the temporal sequence and strength of these associations provide important implications for both treatment and potentially prevention. It is expected that prevention programs that effectively address these risk factors might help to reduce cannabis use and use disorders in adolescents and young adults. At the time this thesis was submitted, the bulk of the material it contains had either already been published or had been submitted for publication in peer-reviewed scientific journals. The great part of the thesis is therefore presented in the format of manuscripts of scientific papers