Identical Twins Reveal Legalizing Recreational Cannabis Has Increased Frequency of Use by 20%

Cannabis Use Increases by 20% After Legalization

According to a new study published in the scientific journal Addiction, the legalization of recreational cannabis in U.S. states appears to have caused a 20% average increase in cannabis use frequency in those states.

The research assessed the effects of recreational cannabis legalization in a large sample of adult identical twins. Of particular interest were the 111 identical twin pairs in which one twin lived in a state with a different recreational cannabis policy to the other. Compared to research on unrelated people, twin studies provide exceptionally well-matched controls for one another and enable more precise estimation of the causal effect of recreational legalization.

Study participants included 1,425 individuals living in states with legal recreational cannabis use and 1,997 living in states in which recreational cannabis use is illegal. Looking at all of them, the study found a ~24% increase in mean cannabis use frequency attributable to legalization. Looking at just the identical twins living in states with different policies, the twin living in a ‘legal’ state used cannabis ~20% more frequently than their co-twin living in an ‘illegal’ state. These results indicate that recreational legalization caused an increase in cannabis use.

Cannabis is the most commonly used drug in the United States that is federally illegal. It is also an addictive substance that is associated with negative health and psychosocial outcomes. Before 2014, cannabis could not be legally bought or sold for recreational purposes anywhere in the United States. However, by early 2022, over 141 million Americans lived in a state where recreational cannabis was legal. 

Reference: “Impacts of recreational cannabis legalization on cannabis use: a longitudinal discordant twin study” by Stephanie M. Zellers, J. Megan Ross, Gretchen R. B. Saunders, Jarrod M. Ellingson, Jacob E. Anderson, Robin P. Corley, William Iacono, John K. Hewitt, Christian J. Hopfer, Matt K. McGue and Scott Vrieze, 24 August 2022, Addiction.
DOI: 10.1111/add.16016

Funding: This work was supported by National Institute on Drug Abuse, National Institute of Mental Health, and National Institute on Alcohol Abuse and Alcoholism of the National Institutes of Health under awards numbers R01DA042755, U01DA046413, R37DA005147, R01DA013240, R01DA036216, R01DA037904, K24DA032555, R01DA035804, P60DA011015, R01DA012845, R01DA038065, R01AA023974, R37AA009367, and R01MH066140. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health

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