Washington State teens are using more marijuana since the state legalized it in 2012, according to a UC Davis and Columbia University study.
The research looked at marijuana usage among teens in Washington and Colorado. They were the first two states to legalize recreational use in the country.
Among 8th and 10th graders in Washington, marijuana use increased by two and four percent, respectively.
Their perception of marijuana's harmful effects decreased by 14 percent for 8th graders and 16 percent for 10th graders.
"Our findings suggest that legalization of recreational use reduced stigma and perceptions of risk associated with marijuana use. And we think that that shift in social norms regarding marijuana could have increase marijuana use among adolescents," says Magdalena Cerda, co-author of the study and an associate professor at UC Davis School of Medicine.
But there was no change in use or perceived harm among teens in Colorado.
Researchers say the difference between teenagers in the two states may be due to Colorado's medical marijuana industry. They say legalization of medical marijuana may have reduced the stigma associated with pot.
California voters legalized recreational use of marijuana in November.
"As states like California legalize recreational use, they should also be thinking about investing dollars in evidence based substance abuse prevention programs, particularly targeted toward adolescents."
Cerda also says California should track teens' attitudes toward marijuana and their use of marijuana.