Washington remains an undefeated cannabis champion on the world stage, and now we have the data to prove it.

A new study from the University of Puget Sound and the University of Washington suggests that Puget Sound, Washington—whose population center is Seattle—may have the highest level of cannabis consumption in the world, beating out the cannabis tourism destination of Amsterdam.

Drug Testing Western Washington’s Sewage

Researchers on the study analyzed a Western Washington city’s public wastewater from 2013 to 2016, looking for THC-COOH, the human byproduct of cannabis’s main active ingredient, THC.

By measuring THC-COOH in thousands of raw sewage samples, researchers hoped to track cannabis consumption levels in the state and see if the legal cannabis market displaced the illegal market.

Washington began retail cannabis sales in August 2014, so researchers were particularly interested in learning how the new laws were affecting cannabis use. Time for Legal Cannabis? Leafly Locates Your Local Store Menus TRY LEAFLY FINDER Most studies on cannabis consumption levels rely on self-reported survey data, which isn’t always reliable due to misreporting. This can be even more unreliable when collecting information about illegal cannabis activity, since respondents may have a strong motive to hide their use.

“These findings suggest that many established users switched from the illegal to the legal market.”

But this new study performed chemical analysis of wastewater from two treatment plants that serviced “a municipality of 200,000 people”—possibly Tacoma, Washington, which has a population of roughly 222,000—to discover how much THC is being consumed by residents there. Researchers then generalized the sample findings to all of Washington. By examining the traces of THC-COOH left behind, scientists were able to track population-level changes in cannabis consumption during the studied period. This measure, generally seen as more accurate than survey data, was able to give big insights into cannabis use in the state.

Insight #1: Cannabis Use Doubled From 2013 to 2016

Published in the journal Addiction June 18, the study, titled “Using wastewater‐based analysis to monitor the effects of legalized retail sales on cannabis consumption in Washington State, USA,” found that cannabis consumption did go up with legal cannabis.

THC‐COOH levels increased by 9% per quarter, “suggesting a doubling” in THC consumption among Western Washington residents between Dec. 1, 2013 and Dec. 1, 2016.

Insight #2: Legal Cannabis Supplanted the Illicit Market

Researchers also think legal cannabis made a huge dent in the illicit market.

That’s because they compared their sewage data with Washington State Liquor and Cannabis Board data on how much THC licensed stores were selling, beginning in 2014.

They found that while THC use was going up 9% per quarter in the population, legal THC sales increased 70% per quarter between August 2014 and December 2016. Researchers say this shows legal cannabis biting into what would have been illicit market sales.

“Given that wastewater represents a total population measure, these findings suggest that many established users switched from the illegal to the legal market,” write the study’s authors, a team from the University of Puget Sound and the University of Washington.

This is great news for those who were hoping legalization would allow cannabis consumers access to safe, lab-tested cannabis.

Insight #3: Western Washington Leads Globe in THC Use

But the study also turned up another result. Lead researcher on the project, Dr. Daniel A. Burgard, says that the data also shows Western Washington has the highest levels of cannabis consumption recorded worldwide.

“We are part of an international study … with 60 to 80 other cities around the world,” he explains. “And according to wastewater, the Puget Sound area has the highest cannabis use per capita, even over Amsterdam.”

While future research could reveal other areas with even higher cannabis consumption, Puget Sound holds the current world record.

Emily Earlenbaugh Dr. Emily Earlenbaugh is a cannabis writer and educator. She is the Director of Education for Mindful Cannabis Consulting, where she teaches patients how to find the cannabis options that work best for them. She regularly writes about cannabis science and culture for publications like Cannabis Now Magazine, SF Chronicle’s GreenState, HelloMD, and Big Buds Magazine. Emily has a doctorate in philosophy of science from UC Davis. View Emily Earlenbaugh's articles