Researchers Must Be Careful in Evaluating Effects of Marijuana Legalization

A researcher who has been part of teams for multiple studies of youth marijuana use in Washington state following legalization of recreational use urges caution in making any conclusive statements about how legalization is affecting young people's behavior.

It will take the completion of studies using state-representative samples in states that have and haven't moved to marijuana legalization to be able to draw firm conclusions, says Magdalena Cerdá, a faculty member in the Department of Population Health at New York University Langone Health. While such research is in progress, Cerdá tells Addiction Professional, other studies that already have been completed do not paint a full picture.

The latest research using Washington state data, published online Dec. 21 in JAMA Pediatrics, analyzed results from the Washington Healthy Youth Survey and found some decreases in youth use post-legalization, in contrast to earlier results that had been based on national Monitoring the Future survey data. Cerdá says, however, “I don't think we can conclude anything new" about the impact of marijuana legalization.

In fact, Cerdá and her colleagues wrote in the latest paper, “It is too soon to know the long-term influence that cannabis legalization will have on the prevalence of its use by youths.”

Other organizations participating in the latest study were the RAND Corporation, the Washington State Liquor and Cannabis Board and the Oregon Public Health Division. Washington voters approved marijuana legalization for recreational use in 2012, with commercialization beginning in the state in 2014.

Comparing the surveys

The earlier study based on Monitoring the Future data was published in JAMA Pediatrics in 2017. It found increases in marijuana use among 8th- and 10th-graders in Washington relative to use in states that had not legalized marijuana. The increases were not seen in 12th-graders.

However, Monitoring the Future's national numbers do not offer state-representative samples for each state, researchers involved in the latest study point out. The new study relied instead on data from the Washington Healthy Youth Survey, which reports on trends in the same age groups that Monitoring the Future examines and is the state's primary information source on young people's health behaviors.

The researchers looked at results from the 2014 and 2016 state surveys and compared them to results from pre-legalization surveys conducted in 2010 and 2012. They found statistically significant decreases in prevalence of marijuana use among 8th- and 10th-graders post-legalization, with no change in prevalence among 12th-graders.

The results indicate that state-representative samples can lead to wholly different numbers from surveys that do not offer state-representative numbers, Cerdá said.

The researchers added that other factors, such as the presence of retail outlets and advertising in the communities where youths reside, need to be explored as potentially more important drivers of youth behavior than a substance's legal status.

“Kids don't care about what happens in the state capitol, they care about what happens in their own neighborhoods,” study co-author Rosalie Liccardo Pacula, co-director of the RAND Drug Policy Research Center, said in a news release. “Commercialization in local neighborhoods is likely to be more important than changes in the law.”