Developmental trajectories of alcohol and cannabis concurrent use in a nationally representative sample of United States youths.

Background: Previous studies have identified common trajectories of single type substance use over the course of adolescence; however, no study to date has examined joint trajectories of cannabis and alcohol concurrent use. Given that expansion of legal cannabis has increased availability, it is important to understand patterns of concurrent use in adolescents and factors that place male and female youth at risk for harmful trajectories of concurrent use. The current study sought to identify joint trajectories of cannabis and alcohol use - and predictors of harmful use trajectories - among male and female adolescents.

Method: We used 4 waves of data from 6997 early adolescent participants (age 12-14 years at Wave 1) in the Population Assessment of Tobacco and Health, a nationally representative longitudinal study in the United States. Participants reported their cannabis and alcohol use reassessed yearly for 5 years (2013-2018). We used joint trajectory growth mixture modeling to identify trajectory groups as defined by changes in alcohol and cannabis use over time.

Results: Five classes of alcohol and cannabis concurrent use trajectories were identified. Both internalizing and externalizing symptoms at Wave 1 increased the odds of membership in trajectory groups characterized by more harmful use trajectories. Internalizing symptomatology was a stronger predictor of membership in escalating use trajectories among girls, whereas externalizing symptomatology stronger predictor among boys.

Conclusions: These findings underscore the utility of jointly considering alcohol and cannabis use when describing common developmental trajectories of use and identifying risk factors for trajectories characterized by harmful use.