Following 2012 legalization of retail marijuana sale to adults in Washington, past 30–day marijuana use decreased or remained stable through 2016 among King County students in grades 6, 8, 10, and 12. Among grade 10 students, the decline in use occurred among males while the rate among females remained steady. Use of alcohol or other substances was four times as frequent among marijuana users as among nonusers.

Use of marijuana at an early age can affect memory, school performance, attention, and learning; conclusions have been mixed regarding its impact on mental health conditions, including psychosis, depression, and anxiety (1–3). Medical marijuana has been legal in Washington since 1998, and in 2012, voters approved the retail sale of marijuana for recreational use to persons aged ≥21 years. The first retail stores opened for business in July 2014. As more states legalize marijuana use by adults aged ≥21 years, the effect of legalization on use by youths will be important to monitor. To guide planning of activities aimed at reducing marijuana use by youths and to inform ongoing policy development, Public Health—Seattle & King County assessed trends and characteristics of past 30–day marijuana use among King County, Washington, public school students in grades 6, 8, 10, and 12. This report used biennial data for 2004–2016 from the Washington State Healthy Youth Survey. Among grade 6 students there was a decreasing trend in self-reported past 30–day marijuana use from 2004 to 2016, while the percentage of grade 8 students who had used marijuana during the past 30 days did not change during that period. Among students in grades 10 and 12, self-reported past 30–day use of marijuana increased from 2004 to 2012, then declined from 2012 to 2016. In 2016, the percentage of students with past 30–day marijuana use in King County was 0.6% among grade 6, 4.1% among grade 8, 13.9% among grade 10, and 25.5% among grade 12 students. Among grade 10 students, 24.0% of past 30–day marijuana users also smoked cigarettes, compared with 1.3% of nonusers. From 2004 to 2016 the prevalence of perception of great risk of harm from regular marijuana use decreased across all grades. Continued surveillance using consistent measures is needed to monitor the impact of marijuana legalization and emerging public health issues, given variable legislation approaches among jurisdictions.

The Healthy Youth Survey is a school-based, anonymous, self-administered, cross-sectional survey conducted in the fall of even-numbered years in Washington public schools.* Schools with grades 6, 8, 10, and 12 are randomly selected using a clustered sampling design. Schools not selected for the state sample also can choose to participate in the survey. The survey measures risk behaviors, attitudes, and factors that contribute to youth health and safety, including alcohol, marijuana, tobacco, and other drug use; behaviors that result in unintentional and intentional injuries (e.g., violence); dietary behaviors, and physical activity.

This analysis used data from all participating schools, both sampled and nonsampled, representing all 19 King County school districts for biennial survey years 2004 through 2016 (the most currently available year of data at the time of analysis). King County is the largest metropolitan county in the state. Local jurisdictions have authority to regulate land uses and can impose additional time, place, and manner-of-use restrictions on state licensed businesses; thus, considerable variation in the availability of and restrictions on retail marijuana exists across the 39 cities in King County, including Seattle.

Survey response rates varied by grade and survey year, with higher rates in more recent surveys.† During 2004–2016, King County response rates ranged from 60%–80% for grades 6 and 8; 50%–70% for grade 10; and 40%–50% for grade 12. For the 2016 survey, response rates for King County were 80% for grades 6 and 8, 70% for grade 10, 40% for grade 12, and 67% for all grades combined.

Data representing substance use, perception of great risk of harm, risky behaviors, and factors associated with marijuana use were categorized dichotomously. Past 30–day marijuana use was considered use on 1 or more days during the past 30 days. Perceived great risk of harm associated with regular marijuana use (more than one or two times per week) was categorized dichotomously as great risk versus all other options combined (moderate, slight, and no perceived risk). Past 30–day use of alcohol, cigarettes, and electronic cigarettes/vape pens was considered use on 1 or more days in the past 30 days, past 30–day risky driving and riding behaviors,§ were considered one or more occurrences during the past 30 days and past binge drinking¶ was over a 2-week period.

Dichotomous factors generally reported to be associated with other substance use (4) were examined for marijuana use; these factors included whether students’ parents had talked about not using marijuana, use by one or more best friends or by a member in the youth’s household, and having been bullied one or more times in the past month.** Stata survey software (version 13; StataCorp) was used to generate percentage estimates and corresponding 95% confidence intervals (CIs). To account for differential participation among school districts across survey years, percentage estimates were weighted to the school district total enrollment by grade and sex, with the final weights adjusted to sum to the county total public school enrollment by grade and sex. Joinpoint trend analysis software (https://surveillance.cancer.gov/joinpoint/external icon) was used to evaluate statistical significance of trends in survey-weighted percentage estimates by grade and sex. Analyses of trends by sex and examination of factors associated with past 30–day use were restricted to grade 10 students as a result of grade-specific sampling and the need for adequate response rates to accommodate a robust analysis.

During 2004–2016, the prevalence of reported past 30–day marijuana use was lowest among students in grade 6 and increased with school grade level (Figure 1). In 2016, past 30–day marijuana use was reported by 0.6% (CI = 0.4–0.7) of grade 6 students, 4.1% (CI = 3.5–4.8) of grade 8 students, 13.9% (CI = 12.6–15.3) of grade 10 students, and 25.5% (CI = 23.7–27.4) of grade 12 students in King County. Among students in grade 6, past 30–day marijuana use declined significantly, from 1.3% in 2004 to 0.6% in 2016. There was no statistically significant trend among students in grade 8; however, among students in grades 10 and 12, past 30–day use increased from 2004 to 2012, and then declined. Across all grades, the percentage of students reporting great risk of harm from regular marijuana use declined over the survey period, with the lowest perceived great risk of harm reported among older students in all years. In 2016, 26.7% (CI = 25.0–28.5) of students in grade 12 perceived great risk of harm from regular marijuana use, whereas 53.3% (CI = 50.5–56.1) reported this perception in 2004.

Among male students in grade 10, past 30–day marijuana use increased from 17.6% in 2004 to 21.4% in 2010 and subsequently declined to 13.5% in 2016 (Figure 2). Among female students in grade 10, there was no change in the prevalence of past 30–day use, which remained approximately 16% during this period. In 2016, there was no significant difference in past 30–day marijuana use between male and female students in grade 10.

Among past 30–day marijuana users in grade 10, 42.8% reported living with someone who uses marijuana, 88.5% reported having at least one best friend who used marijuana, and 26.3% reported having been bullied at least once in the past 30 days; these prevalences were higher than those among grade 10 nonusers (12.8%, 28.3%, and 16.5%, respectively) (Table). Among grade 10 marijuana users, 92.5% reported that it was not very hard to obtain marijuana, compared with 56.7% of nonusers. No parental discussion about marijuana during the past year was reported by similar percentages of past 30–day marijuana users (39.2%) and nonusers (39.8%).

Among grade 10 students, prevalence of past 30–day use of other substances was four times higher among those who had used marijuana in the past 30 days than among those who had not. Among marijuana users, the prevalences of past 30–day use of other substances were as follows; alcohol (67.0%), cigarettes (24.0%), e-cigarettes or vape pens (43.0%), and of binge drinking (43.5%), compared with 10.3%, 1.3%, 4.0%, and 3.7% among nonusers, respectively. Among grade 10 marijuana users, 36% reported driving within 3 hours of using marijuana at least once in the past month.