Frequency of cannabis use
Figure 1 demonstrates that there were a greater proportion of daily cannabis use cases (54.2%) than daily methamphetamines (12.6%) or alcohol use cases (16.8%). Similarly, cases of past month cannabis use were more frequent (66.6%) than for methamphetamines (29.5%) or alcohol (53.1%). Youth workers indicated that 48.2% of the young people they were working with at the time of the census could be considered dependent on cannabis based on their clinical experience, and cannabis was the primary drug of concern in 52.3% of cases. This was a greater proportion of young clients than were dependent on methamphetamines or alcohol (13.0 and 10.9% respectively), or for whom methamphetamines or alcohol were the primary drug of concern (31.2 and 16.5% respectively). Daily cannabis use was not disproportionately represented by gender, Χ2(2) = 2.50, p = .287, Cramer’s V = .06, sexual orientation, Χ2(1) = 0.22, p = .639, Cramer’s V = .02, Aboriginal and Torres Strait Islander, Χ2(1) = 1.03, p = .310, Cramer’s V = .04, or asylum-seeker demographics, Χ2(1) = 0.48, p = .489, Cramer’s V = .02.
Overlap in daily or almost daily substance use was present, with 14.1% of young people engaging in daily cannabis also using methamphetamines daily, Spearman’s ρ(n = 823) = .05, p = .162, whilst 20.4% also used alcohol, Spearman’s ρ(n = 823) = .11, p = .002. In the past month, 33.0% of young people engaging in daily cannabis use had used methamphetamines, Spearman’s ρ(n = 823) = .08, p = .019, whilst 59.6% had also used alcohol, Spearman’s ρ(n = 823) = .14, p < .001. With regards to dependence, 84.8% of young people engaging in daily cannabis use were rated as dependent on cannabis, Spearman’s ρ(n = 823) = .80, p > .001, whilst 13.0% were dependent on methamphetamines, Spearman’s ρ(n = 823) < .01, p = .998, and 12.3% were dependent on alcohol, Spearman’s ρ(n = 823) = .05, p = .163. Cannabis was the primary substance of concern for 65.2% of young people engaging in daily cannabis use, with methamphetamines and alcohol the primary substance of concern for 19.1 and 7.2% respectively, Χ2(1) = 148.59, p > .001, Cramer’s V = .46.
Comparison of substance use categories
A visual comparison of group numbers is displayed in Fig. 2. There were significantly more cases of daily cannabis use than there were daily use cases of cannabis plus other substances, methamphetamines only, or alcohol only, Χ2(4) = 432.58, p > .001, supporting the first hypothesis. Figure 3a compares psychosocial complexity factors proportionately across the five substance use categories. Young people using cannabis daily were significantly more likely than young people not engaging in daily substance use to experience mental health problems, Χ2(1) = 7.32, p = .007, Cramer’s V = .12, employment problems, Χ2(1) = 4.63, p = .032, Cramer’s V = .09, education problems, Χ2(1) = 12.09, p = .001, Cramer’s V = .15, family problems, Χ2(1) = 10.04, p = .002, Cramer’s V = .13, and housing problems, Χ2(1) = 4.83, p = .028, Cramer’s V = .09, supporting the second hypothesis.
Notably, the rate of mental health problems in daily cannabis users included a 37.9% rate of formal diagnosis of a mental health condition. In terms of the past experiences of young people using cannabis daily, the data indicated that these young people had high lifetime histories of neglect (39.9%), emotional abuse (51.1%), physical abuse (39.5%), sexual abuse (15.5%) and being victims of violent crime (24.4%). In relation to family violence more specifically, young people using cannabis were reported to have witnessed family violence in 35.2% of cases, experienced as the victim of family violence in 34.3% and the instigator of family violence in 18.2% of cases. Daily cannabis use was weakly negatively associated with psychological wellbeing, Spearman’s ρ(n = 823) = −.17, p < .001, physical wellbeing, Spearman’s ρ(n = 823) = −.20, p < .001, and overall quality of life, Spearman’s ρ(n = 823) -.19, p < .001. Elaborating on the first hypothesis test, these data further indicate an association between cannabis use and poorer mental and physical health.
To test the third hypothesis, the proportion of each psychosocial complexity factor was compared across cases of daily cannabis use only, daily methamphetamine use only, and daily alcohol use only. No significant differences between the three substances were present for mental health problems, suicidality, employment problems, education problems, criminality, family problems, or housing problems, although young people engaging in daily use of cannabis only were significantly less likely to experience drug-related harm, Χ2(2) = 15.10, p = .001, Cramer’s V = .21. These results support the third hypothesis.
To acknowledge the occurrence of poly-substance use, where an individual may engage in the use of multiple substances on a daily basis, as well as the low number of young people using only methamphetamines or alcohol daily, cases of daily use of cannabis only were compared to cases of daily use of cannabis plus methamphetamines and/or alcohol in terms of psychosocial risk factors. Young people combining daily use of cannabis with daily use of methamphetamines and/or alcohol were significantly more likely to experience drug-related harm, Χ2(1) = 23.89, p < .001, Cramer’s V = .25, suicidality, Χ2(1) = 4.00, p < .045, Cramer’s V = .11, employment problems, Χ2(1) = 5.96, p < .015, Cramer’s V = .12, and education problems, Χ2(1) = 4.89, p < .027, Cramer’s V = .11. No significant differences were present for mental health problems, criminality, family problems, or housing problems.
The above hypothesis tests measured strength of association by analysing the proportion of psychosocial complexity factors relative to group size. This approach allowed inferences to be drawn regarding the likelihood of individual risk based on substance use category, but does not consider the greater prevalence of cannabis use. Figure 3b expresses the same data shown in Fig. 3a as number of participants, rather than as percentage of substance use group. Comparing the mean number of people impacted across the eight psychosocial complexity factors, the amount of young people using cannabis daily affected was 3.49:1 of those using methamphetamines daily, and 2.86:1 of those using alcohol daily, providing a metric of the broader social impact of cannabis use beyond individual risks.
Comparison of 2016 and 2013 cohorts
The 2016 and 2013 cohorts did not significantly differ in age or gender. The proportion of young people using cannabis daily significantly increased to 54.2% in 2016 from 47.5% in 2013, Χ2(1) = 8.09, p = .004, Cramer’s V = .07. There were no significant changesin daily methamphetamine use or daily alcohol use between 2016 and 2013. Cannabis use in the past month remained stable between 2016 and 2013, however there were significantly less cases of use in the past month for both methampetamines, 29.5% versus 34.9% respectively, Χ2(1) = 5.95, p = .015, Cramer’s V = .06, and alcohol, 53.1% versus 63.1% respectively, Χ2(2) = 18.61, p < .001, Cramer’s V = .10. Dependence was not assessed in the 2013 sample. Cannabis was identified as the primary substance of concern in a greater proportion of cases in 2016, up to 52.3% in 2016 from 44.4% in 2013. Methamphetamines as primary concern increased only slightly between cohorts, 31.2% in 2016 versus 30.0% in 2013. The increase in young people for whom cannabis was a concern was a result of displacing individuals for whom alcohol was the primary concern, with alcohol as primary concern decreasing to 16.5% in 2016 from 25.6% in 2013. These frequency changes in primary substance of concern were significant, Χ2(2) = 19.56, p < .001, Cramer’s V = .11.
Investigating the fourth hypothesis, Fig. 4 compares young people using cannabis daily who were assessed in the 2016 ThYNC survey to those assessed in the 2013 ThYNC survey. Young people using cannabis daily in 2016 exhibited significantly higher rates of mental health problems, Χ2(1) = 29.44, p < .001, Cramer’s V = .13, and disengagement from education, Χ2(1) = 4.86, p = .028, Cramer’s V = .05, but a significantly lower rate of current criminal behaviour, Χ2(1) = 13.04, p < .001, Cramer’s V = .09. Rates of drug-related harm, suicide attempt presence, disengagement from employment, family problems, and housing problems remained stable between 2013 and 2016 in young people who were using cannabis daily.