A recent study by the University of Washington Institute on Drug Abuse shows an apparent rise in marijuana use among residents of Washington state since recreational cannabis officially became legal. This is according to a report based on analysis of samples taken from municipal public city wastewater.

Continue Reading Below

The study was conducted by a group led by Chemistry Department Chair Dan Burgard at the University of Puget Sound and the University of Washington.

A Brief History Of Cannabis In Washington

The hybrid cannabis strain Dream Queen in full flower. Dream Queen is a Speed Queen x Blue Dream cross. iStock / tuleerekel
Medical cannabis has been legal in Washington since as early as 1998, at least for those with qualifying medical conditions, although that list has certainly expanded over time. In 2012, Washington voters expressed overwhelming support for making recreational cannabis legal for adults ages 21 and older.

Samples were collected between 2013 and 2016, directly following recreational legalization. Based on the results, researchers determined that marijuana use has increased across the state of Washington.

Continue Reading Below

According to Burgard, “We set out to perform a waste-water 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.”

The Proof Is In The Pudding

The idea is really quite ingenious.

To break it down, the researchers attempted to compare the marijuana content of the wastewater with the growth rate in cannabis sales. They did this by estimating the amount of tetrahydrocannabinol (THC) metabolites found present in the wastewater samples; in this case, researchers were looking at THC-COOH, one of the main metabolites of THC.

Continue Reading Below

Interestingly, between August 2014 and December 2016, the amount of THC-COOH found in the wastewater samples increased by an average of 9% per quarter, while cannabis sales rose by almost 70% per quarter during the same period. 

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

Researchers concluded that the increase in THC metabolite, THC-COOH, reflects the transition from illegal to legal status. It might seem counterintuitive, but relative to the rise in sales, the results of the study were more likely to indicate black-market displacement, one of the major overarching goals of legalization. This could be especially good news for other states on the verge of legalizing.

One of the co-authors of the article and the principle research scientist at University of Washington’s Alcohol and Drug Abuse Institute, Caleb Banta-Green, had this to say following the publication:

“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.”

Continuing on, said Banta-Green, “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.”