Sewage Study in Washington Suggests Recreational Weed Weakened the Black Market
The study also found that Puget Sound, Washington has the highest per-capita rate of cannabis use — even higher than Amsterdam.
Published on June 19, 2019

Proponents of cannabis reform have long argued that adult-use legalization can increase public health and safety by replacing criminal black market pot dealers with regulated legal retailers. Now that cannabis has been legal for as long as five years in some US states, the scientific community is increasingly interested in discovering whether or not legal adult-use markets have actually been able to make good on this promise.

Most research comparing legal and black market weed sales has been done by surveying cannabis users and simply asking them where they bought their weed. But a new study searching for a more objective source of data turned to an unusual source for an answer: the sewers of Washington State.

A team of researchers from the University of Washington and the University of Puget Sound analyzed wastewater samples collected from two water treatment plants in Western Washington between 2013 and 2016. "We set out to perform a wastewater-based analysis that explored 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," explained lead researcher and Puget Sound chemistry department chair Dan Burgard, Science Daily reports.

The study, published on Tuesday in the Addiction journal, found that concentrations of THC-COOH — a metabolite of THC excreted by the body after cannabis consumption – increased by about 9 percent every three months since adult-use pot shops opened their doors in 2014. By comparison, legal weed sales grew by roughly 70 percent per quarter over the same timeframe.

Because legal weed sales were expanding dramatically while overall pot consumption only increased slightly, researchers concluded that cannabis lovers were choosing to buy pot legally rather than from the black market. "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," Burgard said. "This is the strongest statement possible regarding displacement of the illegal market."

Caleb Banta-Green, principal research scientist at the University of Washington's Alcohol and Drug Abuse Institute and co-author of the study, explained how wastewater studies help avoid the limitations of survey-based studies. “Existing measures, particularly surveys are subject to important biases and limitations, including potential changes in self-report as social norms change, as well as very limited information on the amount of THC actually consumed. Wastewater-based estimates help address these limitations."

Using sewage to research recreational drug use may seem like a novel idea, but scientists around the world are increasingly looking to the sewers to source more objective data on drug use. Burgard told KING 5 News that this research is part of an international study “with 60-80 other cities around the world. And according to wastewater, the Puget Sound area has the highest cannabis use per capita, even over Amsterdam.”

Chris Moore
Chris Moore is a New York-based writer who has written for Mass Appeal while also mixing records and producing electronic music.
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