Los Angeles: Marijuana use increased while its perceived harmfulness decreased among teenagers in Washington after the drug was legalised for recreational use by adults, a new study has found.
Researchers from University of California Davis School of Medicine in the US found that there was no change among 12th-graders or among students in the three grades in Colorado after legalisation for adults in Los Angeles.
Washington and Colorado became the first two states in the US to legalise recreational use of marijuana for adults in 2012, followed by handful of other states since.
The potential effect of legalising marijuana for recreational use has been a topic of considerable debate.
Researchers examined the association between legalising recreational use of marijuana for adults in the two states and changes in perception of harmfulness and self-reported adolescent marijuana use before and after legalisation.
They used data from nearly 254,000 students in the eighth, 10th and 12th grades who took part in a national survey of students.
They compared changes prior to recreational marijuana legalisation (2010-2012) with post-legalisation (2013-2015) and with trends in other states that did not legalise recreational marijuana.
In Washington among eighth and 10th-graders, perceived harmfulness declined 14.2% and 16.1% respectively, while marijuana use increased 2.0% and 4.1%, respectively.
Among states that did not legalise recreational marijuana use, perceived harmfulness decreased 4.9% and 7.2% among eighth- and 10th-graders, respectively and marijuana use decreased by 1.3% and 0.9%, respectively, according to the results.
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.
Researchers also offer several potential explanations for the increase in marijuana use among eighth and 10th-graders in Washington, including that legalisation may have increased availability through third-party purchases.
Analysis specific to the states of Washington and Colorado may not be generalisable to the rest of the US.
"A cautious interpretation of the findings suggests investment in evidence-based adolescent substance use prevention programs in any additional states that may legalise recreational marijuana use," the researchers said.
The study was published in the journal JAMA Pediatrics.
