Abstract
BACKGROUND: Binge drinking peaks during emerging adulthood and is associated with negative developmental outcomes. Within-person changes in cannabis use have been shown to coincide with binge drinking; however, whether within-person changes in binge drinking and cannabis use prospectively predict one another and whether these relations vary by age remain unknown. The current study sought to fill these gaps. METHODS: Data come from National Consortium on Alcohol and Neurodevelopment (NCANDA) participants aged 18-25 years reporting alcohol and cannabis use (N = 526). Parallel-process state-trait mixed effect growth models tested whether: (1) binge drinking across emerging adulthood was correlated with cannabis use (random intercepts); (2) steeper growth in binge drinking across emerging adulthood correlated with growth in cannabis use (random slopes); and (3) age-specific, within-person changes in binge drinking/cannabis use reciprocally predicted one another. RESULTS: Across individuals, more frequent binge drinking was correlated with more frequent concurrent cannabis use, and steeper increases in binge drinking were correlated with steeper increases in cannabis use during emerging adulthood. Within-person changes in binge drinking and cannabis use covaried. Within-person increases in cannabis use predicted subsequent increases in binge drinking between ages 18 and 21 years but decreases in binge drinking between ages 24 and 25 years. Within-person changes in binge drinking did not predict subsequent changes in cannabis use during emerging adulthood. CONCLUSIONS: Changes in cannabis use coincided with changes in binge drinking, concurrently and subsequently, particularly between ages 18 and 21 years when changes in cannabis use predicted subsequent increases in binge drinking and ages 24 and 25 years when changes in cannabis use predicted decreases in subsequent binge drinking. Incorporating motivational approaches to reduce cannabis use in alcohol interventions may be efficacious in early emerging adulthood.