Tacoma’s wastewater has the answers in a new cannabis study. Rachelle Abellar

You can learn loads about someone by what comes out of their ass. Dogs know this. People who are into scatology are preoccupied with this. And it turns out that researchers from the University of Washington and the University of Puget Sound realize this, too. After studying our shit, they have discovered one clear trend in our weed habits: We've been consuming it a lot more since its legalization.

These local scientists took samples from Tacoma's wastewater plants from 2013 to 2016 and then analyzed those samples for THC levels. THC is the most common psychoactive chemical found in cannabis, so measuring the quantity of THC in Tacoma's excrement would provide data on how much pot the City of Destiny was ingesting.

The samples showed that the total estimated amount of THC in Tacoma's poop increased 9.4 percent every quarter, according to the study published in peer-reviewed academic journal Addiction. By the end of 2016, Tacoma's residents were consuming twice as much THC as they did in 2013, the last year before legalization. That sizable increase across the population is evidence that legalizing pot increases overall pot usage.

Those rates also show that pot consumers shifted their purchases from the illicit market to the legal market. During that same time period, retail pot sales in Tacoma increased at nearly 70 percent per quarter. Since overall THC consumption was only increasing at 9.4 percent, according to the poop samples, much of that retail sales increase was likely those customers switching from the illegal to the legal market.

The study's authors plan to conduct a follow-up analysis of samples from the same two wastewater plants this year to see if the patterns are maintained into 2019. Caleb Banta-Green, the lead University of Washington author behind the study, said Tacoma's poop data likely reflects Seattle's.

"I think the general patterns are likely similar for the other larger cities/counties in Puget Sound (Seattle, Everett) as they have generally similar illicit drug markets, population demographics, and cannabis markets," Banta-Green said in an e-mail.

Banta-Green, who is also the interim director of UW's Alcohol and Drug Abuse Institute, said he expected that legalization would result in an increase in use, but he did not expect such a large increase.

"I'm surprised to see that much growth. Though upon reflection, now that we see how high potency the products are (and more types of products that are high potency), it's perhaps not such a surprise," Banta-Green said.

Beatriz Carlini, an associate professor at UW's School of Public Health who was not involved in the study, said the information is valuable because it confirms other research showing an increase in usage and a decrease in black market sales. Surveys and sales data all show increases in pot use, but people can lie on survey responses and sales data can be manipulated. Poop won't fall victim to those biases.

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"I think the big novelty and rigor of the paper is more about exploring a new method," Carlini said. "Scientists want to keep developing methods to reconfirm what is already proved from another method. It's not perfect data either, but as you look at different angles of the same question, you have more certainty in what you are saying."

This study isn't without its own limitations, however. Scientists don't have a strong understanding of how bodies process THC, which impacts how specific these estimates can be. And since this data is generalized across the entire population of Tacoma, we don't know if one group of users is just consuming a lot more pot, or if people who didn't consume weed before legalization are now consuming weed, or if it's a combination of both. Either way—pot use is up, and we can see it in our shits.

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