Methods. We used data from the biennial Healthy Youth Surveys from 2004-2016, a statewide, representative sample of 8th, 10th, and 12th grade students (n=161,992). We used multiple logistic regression to conduct trend analyses, and assessed linear and quadratic trends in marijuana use from 2004-2016 for the full sample and stratified on grade level and race/ethnicity, and adjusted for school clustering and sex. Outcomes include any past 30-day marijuana use and frequent use (ie, >6 times in the past 30 days).
Results. There was a modest linear increase in past 30-day marijuana use over the study period (14.7%-16.4%) for the full sample. In analyses stratified by grade, we observed: [1] no change in past 30-day use among 10th graders (17.0%-17.2%, aOR=1.00, 95% CI: 0.98-1.00), [2] a statistically significant linear decrease among 8th graders (9.2%-6.4%, aOR=0.96, 95% CI: 0.94-0.99), and a statistically significant linear increase among 12th graders (19.4%-26.5%, aOR=1.05, 95% CI:1.04-1.07). The increase was particularly high for Hispanic/Latino 12th graders (12.9%-25.2%, 1.12, 95% CI: 1.03-1.07).
Conclusions. Although we observed a modest increase in marijuana use among 8th, 10th, and 12th graders combined in Washington State from 2004-2016, further analyses show that there are differences by grade level; increases in use were found among 12th grade students. Increases in use were largest among Hispanic/Latinos students, and future research should test explanations for this increase, one such explanation is that there were more foreign-born Hispanics in earlier survey years, and being born outside the US is a protective factor in marijuana use. Findings highlight the need for further study into developmental/cultural explanations for age differences to inform prevention and intervention efforts.