Cannabis legalization boosts use by double digits: A first-of-its kind study of twins finds those who live in states where marijuana is legal use it 24% more

Residents of states where cannabis has been legalized use marijuana 24% more frequently than those living in states where it remains illegal, according to new research published today in the journal Addiction.

The study of more than 3,400 adult twins, by researchers at University of Minnesota and University of Colorado, constitutes some of the strongest evidence yet that legalization causes increased use.

It comes at a time when cannabis use is rising nationwide, including during adulthood — a phase of life when individuals have historically tended to cut back.

“Across America, there is a trend toward using more marijuana but we found that the change is bigger in states where it is legal,” said lead author Stephanie Zellers, a recent University of Minnesota graduate who began the research while a PhD student at CU Boulder’s Institute for Behavioral Genetics (IBG).

For the study, Zellers and co-authors at CU Boulder, CU Anschutz Medical Campus and University of Minnesota analyzed data from two large longitudinal twin studies, which have tracked twins since childhood in both states: one housed at IBG and another at the Minnesota Center for Twin Family Research.

Participants were asked how frequently they used cannabis before and after 2014 when Colorado became one of the first states to commence legal sales of recreational marijuana. Recreational cannabis remains illegal in Minnesota. Before 2014, there was little difference in use between states, the study found. After 2014, across all participants, residents of states where recreational use of marijuana was legalized used cannabis 24% more frequently than those in illegal states.

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