After marijuana was legalized for adults in the U.S. state of Washington, younger teens there perceived it to be less harmful and reported using it more, a new study found.

States should consider developing evidence-based prevention programs aimed at adolescents before they legalize the recreational use of marijuana, the researchers said today in JAMA Pediatrics.

"Across the country there has been a decreased perception of risk and an increase in marijuana use among adolescents," lead author Magdalena Cerda, of the University of California, Davis School of Medicine in Sacramento, told Reuters Health by email.

She noted that trying marijuana at a young age is tied to an increased risk of regular use later on. Chronic use of marijuana may be tied to negative outcomes, such as psychosis and poor financial status.

Since 1996, 28 states and Washington D.C. have legalized marijuana for medical purposes, and Colorado and Washington state legalized it for recreational purposes in 2012.

For the new study, the researchers used data from a national survey of 253,902 teens in grades eight, 10 and 12. The survey, conducted between 2010 and 2015, included questions about how harmful adolescents perceived marijuana to be and whether they had used it within the past month.

In Washington state, eighth graders' perception of marijuana's harmfulness fell by about 14 percent from before legalization (2010 to 2012) to afterward (2013 to 2015). Similarly, among 10th graders, the perception of harmfulness decreased about 16 percent.

Additionally, the proportion of kids reporting marijuana use in the previous month rose 2 percent among eighth graders and about 4 percent among 10th graders over that same period.

Those changes were significant when the researchers compared them to states that hadn't legalized recreational marijuana, where teens' perception of harm fell by 5 to 7 percent and their use of the drug only increased about 1 percent.

There were no significant changes in perceived marijuana harmfulness or use among 12th graders in Washington, however. The researchers speculate that older students may already have a fully formed opinion of marijuana.

Additionally, the researchers didn't see any significant before-and-after-legalization differences among students in Colorado. Possibly, they say, this might be because adolescents there were exposed to a robust medical marijuana industry before its recreational use was legalized.

In an editorial accompanying the new study, Dr. Alain Joffe of Johns Hopkins University School of Medicine in Baltimore said research on marijuana is crucial for public policy development.

"Sound public policy should be based on data that are meticulously collected and thoughtfully analyzed," writes Joffe, who is an associate editor of JAMA Pediatrics. "The evolving status of marijuana in the United States provides a critical opportunity for us to do so."

Since 2012, Alaska, California, Maine, Massachusetts, Nevada, Oregon and Washington, D.C. have also approved marijuana for recreational use.

Last year, the American Academy of Pediatrics reaffirmed its opposition to the legalization of marijuana, which it says may be harmful to youths (see http://reut.rs/2hhKNH2).