Legalisation boosts cannabis use but not adverse consequences – study

A comparison of cannabis use in the US has found that legalisation increased use by 24 per cent – but this may be no bad thing.

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

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

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

However, the increase in use was not accompanied by “increased problems” or “adverse consequences”.

The study

For the study, researchers 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 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 cannabis was legalised used cannabis 24 per cent more frequently than those in illegal states.

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

As 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 minimise alternative explanations for results and get at what causes what.

Co-author John Hewitt, a professor in the Department of Psychology and Neuroscience and faculty fellow at IBG, commented: “This is the first study to confirm that the association between legal cannabis and increased use holds within families in genetically identical individuals.

“This makes it much more likely that legalisation does, in itself, result in increased use.”

No increased adverse consequences 

The authors note that it is unlikely that legalisation would cause those who abstained from cannabis before to pick up the habit and preliminary results from their broader ongoing research project suggest increased use may not be a bad thing

Hewit added: “In other analyses, we are finding that this increased use is not accompanied by increased problems, may be associated with less alcohol-related problems and otherwise does not, in general, seem to have adverse consequences.”