In 2012 recreational adult use cannabis was legalized in Washington State. In 2014 the first retail recreational outlets were opened and since then $1.7 billion has been spent in these stores. Have these retail purchases increased the total cannabis market, has it replaced illicit or medical market share, or has it been some combination of these? Survey data has been used to predict the current market share of each of these three sources but no empirical evidence has yet become available through traditional methods. This paper will present wastewater-based epidemiology as a metric to estimate market share of recreational cannabis using levels of THC-COOH, the metabolite of cannabis’s active ingredient, found in Seattle, WA wastewater from June 15 – 30, 2016. 24-h composite samples from a Seattle treatment plant that services 683,000 people yielded THC-COOH metabolite levels of 360 - 810 mg/1000 people per day. These values can be used to back-calculate the consumption of THC per day. Each recreational retail sale is closely tracked by the State and the location and THC content of each product is available. Thus an estimate of market share captured by recreational stores can be calculated by mass balance to create an estimate of the retail market’s contribution to the total amount of THC consumed in the metropolitan area.

Estimating the Seattle, WA legal recreational cannabis market share using wastewater-based epidemiology

253rd National Meeting (2017)

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