Longitudinal Associations between Cannabis Use and Harmful Alcohol Use in Emerging Adulthood in Two States with Differing Cannabis Laws

This research analysed the survey results from 1476 participants from a cross-nationally matched longitudinal study (the International Youth Development Study, IYDS). Participants recruited in Grade 7 (average age 13) in 2002 were followed at average ages 25 (2014) and 29 (2018). Daily cannabis use in Washington State was significantly higher at age 25 (8%) and age 29 (9.9%) compared to Victoria (2.3% and 2.6%).

There was a persistent association between age 25 cannabis use frequency and age 29 harmful alcohol use for both states. Multivariate regression analysis examined whether frequent cannabis use at age 25 cross-nationally predicted harmful alcohol use at age 29, after adjusting for gender, weekly income, employment status, education, and harmful alcohol and tobacco use at age 25. Results: After multivariate adjustment, non-daily cannabis use at age 25 significantly predicted harmful alcohol use at age 29 in Washington State (IRR = 1.36, 95% CI [1.08,1.72], p = 0.009).

Being a non-daily cannabis user at age 25 increased harmful alcohol use at age 29 by 36% in Washington State, while holding all other variables in the model constant. Age 25 cannabis use was a non-significant predictor of age 29 harmful alcohol use in Victoria, however no significant state interaction was found between age 25 cannabis use and age 29 harmful alcohol use. Harmful alcohol use at age 25 was a significant predictor of age 29 harmful alcohol use in both Victoria and Washington State (IRR = 1.12, 95% CI [1.11,1.13], p < 0.001, IRR = 1.11, 95% CI [1.09,1.13], p < 0.001).

Conclusions: These findings demonstrate higher rates of cannabis use in the mid-20s rising slightly into the late 20s in Washington State compared to Victoria. The predictive effect of non-daily cannabis use at age 25 on age 29 harmful alcohol use in Washington State was theorized in terms of non-exclusive cannabis users exploring other substance intoxication. The results of this study assist in understanding how state differences in cannabis legalisation are associated with differing patterns of cannabis and alcohol use into the late 20s.