Longitudinal Effects of Cannabis Use on Suicidal Behaviour in Early Adulthood in Washington State, United States and Victoria, Australia

Abstract This study aimed to investigate the impact of cannabis use on young adult suicidal behaviour. Matched cohorts were compared across two regions: Washington State, USA (where recreational cannabis use was legalised in 2012) and Victoria, Australia (where recreational cannabis use remains illegal). The hypotheses were (1) cannabis use would be more prevalent in Washington State in association with legalisation and (2) cannabis use would predict suicidal behaviour similarly in both regions. Data were drawn from the International Youth Development Study (IYDS) involving 1,470 participants (55.4% female; 726 Victoria, 744 Washington State) surveyed at average ages 25 and 29 in 2014 and 2018, respectively. Participants completed self-report surveys assessing demographic data, cannabis use, other forms of substance use, proximity to cannabis sales outlets, and suicidal behaviour. Data were analysed using multivariate negative binomial regression to identify cannabis use predictors of incident suicidal behaviour and differences between the two regions. Washington State participants reported significantly higher past-year cannabis use compared to Victorian participants at age 25 (in 2014) and lower self-harm at age 29 (2018). Victorian participants reported a reduction in cannabis use from age 25 to 29, while increased use was reported by Washington State participants across the same period. Regression analysis showed higher rates of cannabis use at 25 years and increased cannabis consumption at 29 years predicted increased suicidal behaviour. Suicidal behaviour in 2014 predicted this behaviour in 2018, with higher predictive risk in Washington State. Propensity score analysis confirmed a significant effect of cannabis use on suicidal behaviour. The effect of legalisation indicators (Washington State location, proximity to cannabis stores) on cannabis use was also significant based on propensity analysis. Cannabis was more prevalent in Washington State through the late 20 s. Young adult cannabis use increased the likelihood of suicidal behaviours. Despite increased risk from cannabis use, Washington State maintained lower rates of suicidal behaviour compared to Victoria, emphasising the need to understand multiple determinants of state differences in suicidal behaviour.