Marijuana use has increased but has moved away from the illegal market

The results of a new study from the University of Washington suggest that marijuana use has both increased and substantially shifted away from the illegal market since retail sales began in 2014. The researchers analyzed wastewater samples collected between 2013 and 2016 from two treatment plants that service 200,000 residents in western Washington. "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 …