Cannabis use and related problems decreased in a sample of high-risk young adults since recreational cannabis was legalized in 2018, despite concerns surrounding young adults, a recent study  by McMaster researchers found.

The research, published in JAMA Network Open, followed 619 young adults in Ontario between Feb. 2017 and Feb. 2020, a time frame that includes pre- and post-legalization of recreational cannabis.

The study tracked longitudinal data and followed high-risk young adults who reported regular substance use prior to legalization, including the majority reporting cannabis use.

Researchers pointed out that young adults generally decrease their recreational substance use as they get older — which they call “aging out”.

The study found high-risk young adults showed significant overall decreases in cannabis use and cannabis-related consequences such as low energy or neglecting obligations.

The biggest trends were evident based on pre-legalization patterns.

The most frequent pre-legalization cannabis consumers reported the largest decreases in both use and negative consequences. In contrast, young adults who reported not using cannabis before legalization showed a modest increase in use post-legalization, but no accompanying increases in negative consequences.

“Our study is a great example of why the ongoing monitoring of substance use behaviours is so essential for evidence-based decision making,” explains first author Amanda Doggett, postdoctoral fellow in the Peter Boris Centre for Addictions Research of McMaster University and St. Joseph’s Healthcare Hamilton.

“We were able to measure cannabis use and experiences of cannabis-related consequences among the same people before and after legalization and observed trends that aligned with an expected aging out pattern.”

A limitation of the study is that it is not possible to determine whether the results would have been different in the absence of recreational cannabis legalization.

The findings need to be compared to a control group, or a sample of similar young adults who aren’t from a region where legalization of recreational cannabis occurred, the authors say.

Doggett is collaborating with researchers from the United States to do exactly this type of comparison.

“What is particularly interesting about the group we studied is that many individuals were considered to be ‘high-risk’ in that they were already engaging in cannabis use pre-legalization,” remarks study supervisor James MacKillop, director of the Centre for Medicinal Cannabis Research and a professor in the Department of Psychiatry and Behavioural Neurosciences at McMaster.

“If legalization was expected to have broad impacts on young adults, we should have seen that show up in this higher-risk group; we thought of this group as a ‘canary in the coal mine,’ so to speak.”

The findings bridge a crucial knowledge gap and offer a foundation for future research, the researchers say.

The findings help policymakers understand the broader impacts of cannabis legalization, and provide evidence to inform future public health policy. But they also emphasized the need for caution when interpreting their findings.

“These results reflect changes within the early stages of legalization, before the dramatic proliferation of storefronts, and are in individuals in the developmental window to be naturally growing out of cannabis use,” said MacKillop. “They are by no means definitive about the impacts of legalization on young adult cannabis use.”

Funding for this study was provided by CIHR and MacKillop’s Peter Boris Chair in Addictions Research and Canada Research Chair in Translational Addiction Research.