The legalisation of weed doesn't necessarily lead to increased youth uptake, a new study has found.
In the United States, the legalisation of cannabis in Washington was approved in November 2012. Legal sales began in July 2014.
Under Initiative 502 (I-502), Washington state's legal-cannabis law, reports weighing the costs and benefits of legalisation are required to be conducted periodically.
One such report was released on Saturday, detailing the degree of youth and adult use, treatment admissions and criminal convictions after three years of legal sales.
The study found no increase in cannabis use by young people since legalisation took effect and no increase in admissions for cannabis abuse treatment.
"In my overall appraisal, there's not much evidence I-502 has caused changes in the outcomes we looked at," said lead researcher and author of the report, Adam Darnell.
However, researchers did find adults used more cannabis in some parts of the state.
The report comes after US Attorney-General Jeff Sessions criticised the legalisation of cannabis in Washington.
The findings were based on a biannual survey by the state Department of Health of children in grades six, eight, 10 and 12.
Cannabis use by students in all four grade levels was stable or had fallen slightly since I-502 was enacted, the report found.
Of the 10,835 second-year high school students surveyed last year, 17 percent admitted to having used marijuana in the previous month. In 2006, that figure was 18 percent and in 2010, it was 20 percent.