Complementarity in Daily Marijuana and Alcohol Among Emerging Adults

Objective: The relationship between marijuana and alcohol use among late adolescents was examined as whether marijuana use was related to quantity of alcohol consumed that day, and whether changes in marijuana and alcohol use frequency over 3 years were related. Method: College students (n = 375) reported marijuana and alcohol use for 28 days over 3 years. Results: Within-day analyses showed that more alcohol was consumed on days on which marijuana was used. Co-use varied by alcohol use problems, day of week, and year in study; the relation between marijuana and alcohol use was stronger among those who reported less problematic alcohol use, on weekdays, and as time increased. Daily level co-use relations also differed marginally by gender. At the annual level, there was a marginal relation between changes in use such that increasing use of one substance over time was weakly associated with increased use of the other substance over the same time period. Conclusions: Results add to the emerging conclusion of complementary marijuana and alcohol co-use within a single day, showing it occurs for both women and men, across a full range of marijuana use, and increases with time while being affected by alcohol use problems and day of week. The overall strong co-use relationship highlights risky behavior among late adolescents and supports an emphasis on common underlying substance use causes. However, the different pattern when assessed yearly demonstrates the importance of tailoring the timescale of analysis to the specific substance use question. Public Health Significance Statement Results show that late adolescents making the transition to college co-use marijuana and alcohol within individual days, with marijuana use on a particular day associated with more drinks consumed the same day. The relationship between daily marijuana and alcohol use was weaker among those with more problematic alcohol use, on weekends, and marginally so among men. Marijuana use also showed a stronger relation to alcohol use across the measurement period, suggesting co-use risk increased across the college transition. The consistent evidence of within-day co-use suggests substance use interventions need to explicitly consider motivations for and target co-use.