| Sumario: | Aims: To perform a wastewater‐based analysis to explore the impact of newly legalized retail cannabis sales on its use and to determine if this approach could estimate the size of the legal market place, which began 1 August 2014 in the study area. Design Laboratory study of raw wastewater samples collected and analyzed over the 3‐year period from 2014 to 2016. Setting and Participants: Samples obtained from the two wastewater treatment plants that serviced a municipality of 200 000 people in the state of Washington, USA. Measurements Quantitative analysis of 24‐hour composite influent samples for the metabolite of the active ingredient in cannabis, 11‐nor‐9‐Carboxy‐Δ9‐tetrahydrocannabinol (THC‐COOH) were performed by liquid chromatography coupled to mass spectrometry. Findings Wastewater estimates for THC‐COOH increased by 9% per quarter, suggesting a doubling in cannabis consumption from 1 December 2013 to 31 December 2016. State‐sold THC increased at nearly 70% per quarter, while stores operated from 1 August 2014 to 31 December 2016. Estimating the proportion of the total cannabis market supplied by state‐regulated cannabis from these data is not currently achievable. Conclusion: A wastewater‐based measure of cannabis consumption suggests a significant increase in consumption in Washington, USA following legalization, and that legal sales appear to have displaced a large portion of the illicit market.
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