The number of teens toking since recreational marijuana use was legalized for people 21 and older in Washington state six years ago has declined significantly, according to new research published in the journal Jama Pediatrics. 

To study the effects legalization has had on underage users, researchers analyzed data from the Washington Healthy Youth Surveys – an anonymous, school-based survey of 8th, 10th, and 12th graders – from 2010 to 2012 and then again between 2014 and 2016. They compared this data against a nationally representative Monitoring the Future survey to determine whether there were any differences from other states without legalized recreational marijuana. The main difference in usage was observed among 10th graders (teens around 15 and 16 years old), whose marijuana use declined from almost 20 percent to less than 18 percent in that time frame. Among 8th graders, marijuana use fell from nearly 10 percent to just over 7 percent, while no change was observed in 12th graders, or teens typically around 17 and 18 years old.

Meanwhile, a study published just a few days earlier found that, in general, marijuana consumption in Washington state is about twice as large as previously estimated. According to the Centers for Disease Control and Prevention, marijuana is the most commonly used illegal drug in the US, with an estimated 22 million users every month. Research suggests that about 1 in 10 users will become addicted. For those who begin using before 18, that number jumps to 1 in 6.

While marijuana has many science-backed health benefits when used for medicinal purposes, research has shown that using it while the brain is still undergoing crucial development and growth during teen years can cause problems with learning, emotional well-being, and health. The American Academy of Child and Adolescent Psychiatry notes side effects can have impacts on respiratory health and underlying mental health conditions such as schizophrenia, depression, and anxiety. (They also write that parents suspecting their children may be using marijuana should look for new words and phrases like ‘sparking up,’ ‘420,’ ‘dabbing,’ and ‘shatter.’”)

Though the findings are promising, the researchers note that there is not enough evidence to determine the long-term effects of legalization on those using marijuana under 18.

"These findings do not provide a final answer about how legalization ultimately may influence youth marijuana usage," said study co-author Rosalie Liccardo Pacula in a statement. "A variety of factors may influence the behavior of adolescents and those factors are likely to influence behaviors in different ways over time.”

The authors note that the association between legalization and underage use is a “complex” one, and there is still much to be understood before jumping to conclusions.

“It is too soon to know the long-term influence that cannabis legalization will have on the prevalence of its use by youths,” wrote the authors, concluding that future studies are needed in order to better understand how availability and exposure could impact young users.