Cannabis use hits new highs in US

New research has highlighted there has been a huge rise in the use of cannabis in the US states which have legalised its use.

The study by researchers at the University of Minnesota and University of Colorado found residents of states where cannabis has been legalised use marijuana 24% more frequently than those living in states where it remains illegal.

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

More than 141 million Americans now live in a state with recreationally legal cannabis and, according to the National Institute on Drug Abuse, use among young adults aged 19 to 30 is at an all-time high, with 43% reporting use in the past year and 29% in the last month.

“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 Behavioural Genetics (IBG).

For the study, Zellers and co-authors at CU Boulder, CU Anschutz Medical Campus and University of Minnesota analysed 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 Centre 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.

When specifically comparing identical twins in which one now lives in a state where marijuana is legal and the other lives in a state where it is illegal, those living in the state with legal marijuana used cannabis 20% more frequently, the researchers found.

Because twins share their genes and tend to share socioeconomic status, parental influences and community norms, they provide well-matched controls for each other, enabling researchers to minimize alternative explanations for results and get at what causes what.

“This is the first study to confirm that the association between legal cannabis and increased use holds within families in genetically identical individuals,” said co-author John Hewitt, a professor in the Department of Psychology and Neuroscience and faculty fellow at IBG. “This makes it much more likely that legalisation does, in itself, result in increased use.”

“Typically, what we would expect to see is that people tend to increase use as adolescents and then reduce it as they transition into adult roles, family life and stable jobs,” said Zellers. “Interestingly, we saw escalation, not reduction, in adults.”