Changes in marijuana use across the 2012 Washington State recreational legalization: Is retrospective assessment of use before legalization more accurate? (2018)

OBJECTIVE: The purpose of this study was to evaluate changes in marijuana use prevalence and user characteristics across the 2012 recreational legalization in Washington State. Differences in change estimates between retrospective and contemporaneous pre-legalization measures are compared and considered in relation to potential social acceptability and illegality effects on reporting.
METHOD: Four representative surveys of the Washington State population 18 years and older were conducted by telephone, two in 2014 and two in 2015, which are combined by year for analyses (N = 3,451). Respondents reported their current past-year use frequency and retrospective frequency of use in 2012 before the election in which legalization was passed. They also provided demographic information and details of alcohol use, including simultaneous use with marijuana.
RESULTS: A small and not statistically significant increase of 1.2 percentage points in past-year use prevalence, from 24.3% (22.3-26.5) to 25.6% (23.6-27.6), was found when combining the surveys. No statistically significant change was found in the prevalence of simultaneous use with alcohol, which decreased from 12.9% (11.3-14.7) to 12.6% (11.0-14.4). In contrast, estimates from the National Survey on Drug Use and Health (NSDUH) indicate substantially increased prevalence, from 15.5% (13.8-17.3) in 2010-2012 to 19.1% (16.9-21.4) in 2013-2014, although this change is not statistically significant. Other findings of interest from the Washington State surveys include new users after legalization tending to be older, White, and moderate drinkers who do not use marijuana simultaneously with alcohol.
CONCLUSIONS: A retrospective pre-legalization measure showed only a small increase in marijuana use prevalence in contrast to larger changes found in prospectively assessed use in the NSDUH. Changes in the social acceptability and legal status of marijuana after legalization may have increased reporting of pre-legalization use compared with concurrent assessments.