Marijuana and Other Substance Use after Washington State Recreational Marijuana Legalization: Cohort Comparisons in a Sample of Adolescents

Colorado and Washington State recently passed legislation allowing recreational marijuana use for adults. The potential influence of this legislation on adolescent marijuana and other substance use (e.g., spillover or substitution effects) is uncertain. We capitalize on an ongoing longitudinal study to examine the prevalence of marijuana and other substance use in two annual cohorts of adolescents who experienced the Washington State recreational marijuana law change at different ages. Participants were low-income 8^th graders enrolled in targeted public middle schools in Tacoma, WA and recruited in two cohorts over two consecutive academic years (2010/2011 and 2011/2012). The analysis sample was 238 students who completed a baseline survey in the 8^th grade and a follow-up survey after their 9^th grade year. Self-report survey data on lifetime (at baseline) and past month (at follow-up) marijuana, cigarette, and alcohol use were collected.

Supporting a substitution-effect hypothesis, multivariate multilevel modeling showed that cohort differences in the likelihood of marijuana use were significantly different from those for cigarette and alcohol use at follow-up (adjusting for baseline substance initiation). Marijuana use was slightly higher for the second cohort (11.8%) than the first cohort (6.8%), a difference that was significant at p < .10 (Adjusted Odds Ratio = 2.81). The rates of cigarette and alcohol use were slightly lower in the second cohort (4.1% cigarettes; 8.3% alcohol) than in the first cohort (12.0% cigarettes; 12.4% alcohol).

The law did not appear to have a large impact on adolescent marijuana or other substance use in the short term, although the pattern of use across cohorts suggests that the early stages of legalization may have resulted in increases in marijuana use and decreases in cigarette and alcohol use. While the recreational marijuana legislation passed prior to the second cohort completing its follow-up interview, full implementation of legalization had not gone into effect (e.g., shops had not opened). The analytic technique used in this study will be useful for continued surveillance to monitor potential long-term effects of the law change on youth. Indeed, forthcoming data collection at a longer-term follow-up in this study will be analyzed soon, and results, presented in conjunction with those reported above, will be informative with respect to potential changes as the law plays out in Washington State.