Legalization of Marijuana Has Decreased Black Market Sales

According to the recently published article by the University of Puget Sound and University of Washington researchers, the market of cannabis has changed significantly in the past few decades. Their findings are conferred from the evaluation of a particular population’s urinary samples obtained from the public wastewater sewage system. The wastewater analysis inferred a significant shift as well as a decline in the trade of illegal marijuana following the legalization and retail sale of marijuana since 2014.



Summary of the research

Researchers: Dan Burgard, Jason Williams, Danielle Westerman, et al

Duration of study: 2013-2016

Participants: Wastewater samples were collected from a community of 200,000 residents in Western Washington.

Research Principle: THC-COOH is the metabolic product of psychoactive chemical THC found in cannabis. It is excreted through the renal system and can be detected in the urine of a person who has recently used cannabis for recreational or medical purposes. The researchers aimed to compare the increase or decrease in illicit cannabis use after the legalization of marijuana, thereby assessing the scope of the marketplace for legal marijuana.

Methodology: Raw wastewater samples obtained from two treatment plants were tested to detect levels of drugs and their metabolites such as THC-COOH.

Findings: In between December 2013 to December 2016, THC-COOH levels in wastewater went up by 9% each quarter whereas retail sales of cannabis increased by almost 70% from August 2014 to December 2016.


Despite a significant rise in the retail sale of marijuana, the overall population consumption did not rise at the same rate which indicated a significant switch in the purchase of cannabis by established users from illicit markets to legalized retail marijuana stores.

“Given that wastewater represents a total population measure, these findings suggest that many established users switched very quickly from the illegal to the legal market,” said Burgard, lead researcher, and chemist. “This is the strongest statement possible regarding displacement of the illegal market.”

Wastewater analysis of a population aids in the assessment of the positive or negative outcomes to the legalization of cannabis or other illicit drugs. Although this method is extremely fruitful to analyze overall population trends, it is not valuable for individual analysis of drug use. However, detecting metabolite levels of drugs in a particular area can be used to inversely detect the increase or decline in the severity of drug abuse in a particular area.


1. Daniel A. Burgard, Jason Williams, Danielle Westerman, Rosie Rushing, Riley Carpenter, Addison LaRock, Jane Sadetsky, Jackson Clarke, Heather Fryhle, Melissa Pellman, Caleb J. Banta-Green. Using wastewater-based analysis to monitor the effects of legalized retail sales on cannabis consumption in Washington State, USA. Addiction, 2019; DOI: 10.1111/add.14641



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