Twin Study Find Cannabis Legalization Linked to Increased Marijuana Use in Adults What are the main findings?

Response: We found that, across all individuals, people living in recreational states used cannabis 24% more frequently than individuals living in illegal states. Within identical twin pairs, the twin living in a legal state used cannabis 20% more frequently than their co-twin living in an illegal state. By comparing the effect sizes across all individuals (24%) and just identical twins (20%), we can conclude that cannabis legalization does cause an increase in cannabis frequency in adults, even after controlling for unmeasured shared confounds and earlier cannabis use. What should readers take away from your report?

Response: Our results confirm the effect sizes identified by earlier cross-sectional studies and suggest that recreational cannabis legalization does causally increase cannabis use frequency. It is important for the public to know how cannabis policies may impact adults’ behavior, but also that this study only demonstrates how cannabis use changes as a result of cannabis policies. More work is needed to understand the broader consequences of recreational cannabis policies on other important outcomes like mental and physical health. What recommendations do you have for future research as a result of this study?

Response: We are actively working on projects to evaluate the impact of recreational cannabis legalization on other important outcomes for public health, like addiction, mental health, and psychosocial adjustment. To understand the impacts of recreational cannabis legalization most accurately, it is important to research a broad scope of outcomes, like physical health, mental health, and safety outcomes (ex., DUIs, illicit drug related morbidity and mortality).

Disclosures: We have no competing interests, this work was funded by the National Institutes of Health (NIDA, NIMH, NIAAA). The article and interview content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health

 The article (Open Access) can be accessed at

Full citation for article: Zellers SM, Ross JM, Saunders GRB, Ellingson JM, Anderson JE, Corley RP, Iacono W, Hewitt CJ, McGue MK, and Vrieze S (2022) Impacts of Recreational Cannabis Legalization on Cannabis Use: A Longitudinal Discordant Twin Study. Addiction: doi: 10.1111/add.16016

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Last Updated on August 29, 2022 by