In November 2016, another four states voted to legalize marijuana for recreational use by adults.
But do these laws affect the perception and use of pot among teens?
A new study from UC Davis shows mixed results.
In Washington State, marijuana use among eighth and tenth graders increased two to four percent after it became legal in 2012. And teens' perception that pot is harmful dropped by about 15-percent.
But researchers found no change among students in Colorado, which also legalized recreational marijuana the same year.
During the same time period, marijuana use among teens living in non-legal states dropped by one-percent and their perception of its harmfulness fell by about six percent.
Editor's note: This study was led by researchers at UC Davis and published in 'JAMA Pediatrics'