Prevalence of marijuana and other substance use before and after Washington State's change from legal medical marijuana to legal medical and nonmedical marijuana: Cohort comparisons in a sample of adolescents.

Background: A growing number of states have new legislation extending prior legalization of medical marijuana by allowing nonmedical marijuana use for adults. The potential influence of this change in legislation on adolescent marijuana and other substance use (e.g., spillover or substitution effects) is uncertain. We capitalize on an ongoing study to explore the prevalence of marijuana and other substance use in 2 cohorts of adolescents who experienced the nonmedical marijuana law change in Washington State at different ages.

Methods: Participants were 8th graders enrolled in targeted Tacoma, Washington public schools and recruited in 2 consecutive annual cohorts. The analysis sample was 238 students who completed a baseline survey in the 8th grade and a follow-up survey after the 9th grade. Between the 2 assessments, the second cohort experienced the Washington State nonmedical marijuana law change, whereas the first cohort did not. Self-report survey data on lifetime and past-month marijuana, cigarette, and alcohol use were collected.

Results: 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 higher for the second cohort than the first cohort, but this difference was not statistically significant. Rates of cigarette and alcohol use were slightly lower in the second cohort than in the first cohort.

Conclusions: This exploratory study found that marijuana use was more prevalent among teens shortly after the transition from medical marijuana legalization only to medical and nonmedical marijuana legalization, although the difference between cohorts was not statistically significant. The findings also provided some evidence of substitution effects. The analytic technique used here may be useful for examining potential long-term effects of nonmedical marijuana laws on adolescent marijuana use and substitution or spillover effects in future studies.