Methods: Biannual data for 6th, 8th and 11th graders from the Student Wellness Survey (SWS) in 36 Oregon counties from 2010 to 2016 (N=191,543) were analyzed to assess relative changes in past-30-day marijuana use and marijuana-related beliefs (perceived availability of marijuana, perceived risk of marijuana use, perceived parent approval of marijuana use) in counties that allowed recreational marijuana sales in 2015 compared to counties that did not. Analyses were also conducted with the 2016 SWS data (n=50,791) determine whether county-level associations between allowing recreational marijuana sales and outlet densities with marijuana use could be accounted for by perceived availability, risk of harm, and parents’ approval of marijuana use.
Results: Higher rates of past-30-day marijuana use and more favorable beliefs about marijuana use were observed in counties that allow recreational marijuana sales. The prevalence of past-30-day marijuana use declined from 2010 to 2014, but increased from 2014 to 2016 in both counties that did and did not allow recreational marijuana sales. There were parallel increases in perceived availability of marijuana and perceived parent approval of marijuana use from 2014 to 2016 and a steady decline in perceived risk of marijuana use from 2010 to 2016. Analyses of the 2016 SWS data suggested that the associations between allowing recreational marijuana sales and past-30-day marijuana use could be accounted for by density of licensed marijuana retail outlets and marijuana-related beliefs.
Conclusions: There was a statewide increase in the prevalence of marijuana use and in beliefs favorable to marijuana use after legalization of recreational marijuana sales in 2015. Increasing retail availability of marijuana for recreational use may increase marijuana use among adolescents by increasing beliefs favorable towards marijuana use.