Calendar Month Variation in Alcohol and Marijuana Use in a Community Sample of Young Adults.

OBJECTIVE: Alcohol use among adults varies by season, and drinking among 4-year college students has been linked to holidays and the academic calendar. Little research has assessed calendar-related variation in marijuana use. We examined calendar month variation in alcohol and marijuana use among a sample of young adults that was diverse with respect to college status and educational attainment. METHOD: Data were drawn from a community sample of 761 individuals (57% female; ages 18-23 at enrollment). Participants were surveyed monthly for 24 consecutive months. Multilevel models accounted for nesting of monthly data within individuals. RESULTS: All measures of substance use showed evidence of variation by calendar month, with the exception of hours high in months when marijuana was used. Drinks per week and peak number of drinks were relatively elevated in summer months and in October and December; the prevalence of heavy episodic drinking was also elevated in October and December. Attending a 4-year college was associated with more drinking than not being enrolled in a 4-year college, regardless of whether a 4-year degree had been obtained. Students in a 4-year college reported more drinks per week than those not in a 4-year college in September and October, whereas the peak number of drinks converged across educational status in the summer months and in December. Across educational statuses, marijuana use was highest in April and December. CONCLUSIONS: Findings may guide the targeting and timing of substance use prevention programs by focusing efforts before and during higher risk months.