If you want to stop drug dealers from pushing cannabis, new research suggests, legalizing its recreational use definitely works.

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Researchers from the University of Puget Sound and the University of Washington report that legalizing recreational cannabis can increase the use of the drug but, more crucially, shifts purchases from the illicit market to legal outlets. The results are based on an analysis of wastewater samples from one Western Washington population center over several years.

Whitening market

“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 marketplace,” says Dan Burgard, chair of the chemistry department at Puget Sound and lead author of the paper.

Burgard’s team analyzed samples of wastewater collected between 2013 and 2016 from two treatment plants in Western Washington. Collectively, the two plants serve around two hundred thousand locals. The researchers tested samples from 387 days spread over three years. The team relied on a new analysis method for faster and more accurate assessments of illicit drug consumption compared to existing measures.

Raw wastewater samples were collected at treatment plants and analyzed for drugs and their metabolites — the byproducts created by our bodies as they process a given substance — at extremely low concentrations (parts per billion or parts per trillion levels). These figures were used to track drug consumption trends, both legal and illegal. While the approach doesn’t allow for the team to track individual users’ habits, they do show overall trends. For example, the concentration of the metabolites can be used to calculate the actual number of doses of a drug used in a particular area.

Based on the readings, the team estimates that THC-COOH — THC, the psychoactive component in cannabis — in the wastewater increased by 9% per quarter, on average, from December 2013 to December 2016. During roughly the same time, August 2014 to December 2016, recreational cannabis sales increased at around 70% per quarter on average. In other words, the increase in legal sales of cannabis far outstripped the rise in THC-COOH in raw wastewater — showing that users are making a massive shift towards legal avenues of getting their fix.

“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,” says Burgard. “This is the strongest statement possible regarding displacement of the illegal market.”

The findings come to flesh-out our understanding of how the legalization of cannabis for recreational purposes impact both its use and the illicit market. In the past six years nine U.S. states (Colorado, Washington, Alaska, Oregon, Nevada, California, Maine, Massachusetts, Vermont, Michigan, and the District of Columbia) have legalized the adult use of recreational cannabis, as did the countries of Uruguay and Canada, making such data valuable to policymakers there and in other areas of the world. Caleb Banta-Green, interim director and principal research scientist at the University of Washington’s Alcohol and Drug Abuse Institute and co-author of the paper, says:

“This project was designed to aid the understanding of how the sales of adult recreational cannabis impact its total consumption within a population. We believe this will be a valuable tool for local, state, national and international policy makers as they assess and consider Washington’s recreational cannabis law.”

“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.”

One of the main findings of this study, the team explains, is that legalization does (at least in part) eliminate black market sales of marijuana.

The paper “Using wastewater-based analysis to monitor the effects of legalized retail sales on cannabis consumption in Washington State, USA” has been published in the journal Addiction.