Did marijuana legalization in Washington State reduce racial disparities in adult marijuana arrests?

<i>Background</i>: One justification for marijuana legalization has been to reduce existing disparities in marijuana-related arrests for African Americans. <i>Objective</i>: Describe changes in adult marijuana arrest rates and disparities in rates for African Americans in Washington State (WA) after legalization of possession of small amounts of marijuana for 21+ year olds in December 2012, and after marijuana retail market opening in July 2014. <i>Methods</i>: We used 2012-2015 National Incident Based Reporting System data to identify marijuana-related arrests. Negative binomial regression models were fit to examine monthly marijuana arrest rates over time, and to test for differences between African Americans and Whites, adjusting for age and sex. <i>Results</i>: Among those 21+ years old overall, marijuana arrest rates were dramatically lower after legalization of possession, and did not change significantly after the retail market opened. The marijuana arrest rates for African Americans did drop markedly and the absolute disparities decreased, but the <i>relative</i> disparities grew: from a rate 2.5 times higher than Whites to 5 times higher after the retail market opened. Among 18-20 year olds overall, marijuana arrest rates dropped, but not as dramatically as among older adults; the absolute disparities decreased, but the relative disparities did not change significantly. <i>Conclusions</i>: Marijuana arrest rates among both African American and White adults decreased significantly with legalization of possession, and stayed at a dramatically lower rate after the marijuana retail market opened. However, relative disparities in marijuana arrest rates for African Americans increased for those of legal age, and remained unchanged for younger adults.