"Older adolescents may also have had their attitudes and beliefs about marijuana formed before recreational marijuana use was legalized, making it less likely their use would change after legalization".
However, many marijuana legalization opponents may take issue with the fact that the use of marijuana is increasing at a much higher rate in Washington.
"A cautious interpretation of the findings suggests investment in evidence-based adolescent substance use prevention programs in any additional states that may legalize recreational marijuana use", the authors write.
In 2012, Washington and Colorado became the first two states to legalize recreational marijuana use.
In general, teenagers have been perceiving weed as being less harmful across the country.
There was also a significant difference-in-difference in perceived harmfulness among eighth and tenth graders in Washington compared to their peers in states without recreational marijuana laws (-9.3%, P=0.01 and -9.0%, P=0.02, respectively).
By 2013 to 2015, 14 percent fewer 8th graders perceived marijuana to be harmful.
No changes were seen in perceived harmfulness or marijuana use among Washington 12th-grades or students in the three grades in Colorado, for which researchers offer several explanations in their article.
After marijuana was legalized for adults in the US state of Washington, younger teens there perceived it to be less harmful and reported using it more, a new study found.
Teens in Colorado also experienced a decrease in perceived health risks surrounding marijuana, but it was somewhat less dramatic - about 3 percent for 8th graders and 11 percent for 10th graders, according to the report. One other study had previously found a non-significant difference in higher marijuana use among eighth-graders in Washington following these laws.
The counter at Studio A64 a cannabis club in Colorado Springs
A second editorial by Wayne Hall, PhD, of University of Queensland in Australia, and Megan Weier, BPsySci, of King's College London in the United Kingdom, addressed the challenge that health educators in states with recreational marijuana laws will face, as "increased promotion of marijuana to adults will reach adolescents" from an industry who "wants to grow its market by increasing regular use and downplaying its health risks".
They note that Colorado had a well developed medical marijuana dispensary system with substantial advertising prior to legalization, while Washington had substantially less commercialization and advertising for its medical pot stores.
The researchers find that recent pot use among Washington eighth and 10th-grade students increased between 2013 and 2015, while use decreased among that age group in states where recreational pot is illegal. NORML advocates for reform of marijuana laws.
Cerda explains that researchers compared perceptions of use and actual use before and after legalization.
"The changes in use seem to appear small and inconsistent, particularly in light of comparable changes in states where prohibition still reigns", said Earleywine, a professor of psychology at the State University of NY at Albany. It compares teens attitudes towards pot before and after legalization.
She told Reuters Health that trying marijuana at a young age is tied to an increased risk of regular use later on.
Recreational legalization can increase the amount of pot available to kids, and at the same time give the perception that there's nothing wrong with using marijuana, Krakower noted.
"The challenge for health educators will be in acknowledging that the acute adverse effects of marijuana use are modest by comparison with those of alcohol or heroin while persuading young people that they can experience adverse effects, especially if they begin to use in their teens and use daily throughout young adult life", says Hall. "Combine that with a legalized market where you can readily buy it and it will be easier to obtain, and children will be more likely to use it".
The Monitoring the Future Study is supported by the National Institute on Drug Abuse and the National Institutes of Health.