In December 2012, the possession and private use of limited quantities of marijuana and marijuana products became legal in the state of Washington. At the same time, the state's driving under the influence statutes were amended to include a per se level of 5 ng/mL delta(9)-tetrahydrocannabinol (THC) in whole blood for drivers aged 21 years and older. The aim of this study was to assess the effect of marijuana legalization on the prevalence of marijuana in suspected impaired driving cases. The prevalence of both active THC and its metabolite carboxy-THC detected in such cases pre-legalization was compared with the prevalence post-legalization. In 2009-2012, the average yearly percentage of cases positive for THC and carboxy-THC was 19.1% (range: 18.2-20.2%) and 27.9% (range: 26.3-28.6%), respectively. In 2013, the percentages had significantly increased to 24.9 and 40.0%, respectively (P < 0.05). The median THC concentration over the 5-year period ranged from 5.2 to 6.3 ng/mL, with individual concentrations ranging up to 90 ng/mL. An average of 56% of cases were at or >5 ng/mL over the 5-year period. The prevalence of alcohol and the majority of other drugs in this same population of suspected impaired drivers submitted for testing did not change during this same 5-year period-marijuana was the only drug to show such an increase in frequency. Further, this observed increase remained after the data had been normalized to account for changes in laboratory testing procedures that occurred during this time period. Future studies need be conducted to ascertain whether the observed increase has had any effect on the incidence of crashes, serious injuries and/or traffic fatalities.
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