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Cannabis Use Jumps By 20% in Legal States, Study of Identical Twins Finds

By Alexander Beadle

Published: Sep 05, 2022   
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People in states where recreational cannabis is legal tend to use cannabis, on average, 24% more frequently than their peers in states where use is prohibited, a new study suggests.

Published in the journal Addiction, researchers from the University of Minnesota and University of Colorado used longitudinal survey data from more than 3,400 adult twins – 111 of which were identical – to study recent trends in cannabis use and the factors affecting it.

Based on this new twin study, the researchers believe that legalization is “much more likely” to be the major factor in increasing cannabis use frequency, as opposed to some other genetic or environmental reason.

Adults use cannabis more frequently after legalization

Researchers analyzed data from two large, longitudinal twin studies carried out by the University of Colorado Boulder’s Institute for Behavioral Genetics (IBG) and the Minnesota Center for Twin Family Research. Both studies have tracked the lives of twin pairs in Minnesota and Colorado since birth.

For this study, researchers focused on drug use data taken at each participant’s last assessment before 2014 (the year where Colorado legalized recreational cannabis use) and a second assessment taken post-2018.

They found that, overall, there were no significant differences between those living in Colorado or Minnesota before 2014. However, average cannabis use frequency among those residing in Colorado rose by 24% following legalization.

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

“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,” Zellers continued. “Interestingly, we saw escalation, not reduction, in adults.”

Legalization is the direct cause of this increase, twin study reveals

The researchers were particularly interested in the 111 identical twin pairs who were living in different states with discordant cannabis policies, where one state allowed for legal recreational use and the other did not.

Because such twins share genetic material and will also have similar upbringings in terms of socioeconomic status and parental influence, identical twin studies can give researchers an opportunity to control for many factors that may otherwise complicate direct comparisons.

Here, the groups of twins living in different states represented an opportunity to study the effects of differing cannabis policy. Within this specific subgroup, the researchers found that the twins living in the legal use state also used cannabis around 20% more frequently than their siblings in states where recreational use was not permitted.

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

Interestingly, the researchers also say that follow-up analyses suggest that the increase in average cannabis use frequency was likely reflecting an increased prevalence of recent use among existing cannabis users.

“Our analyses suggest that among individuals who have used in their lifetime, cannabis legalization may cause increased likelihood of recent use, but cannabis legalization is unlikely to cause initiation in individuals who were lifetime abstainers prior to legalization,” the researchers wrote.

Legalization and cannabis use

Cannabis use among young adults is at an all-time high, according to a recent report from the University of Michigan, sponsored by the National Institute on Drug Abuse. The report found that 42.6% of young adults aged 19 to 30 claimed to have used cannabis in the past 12 months, with 28.5% reporting use in the past 30 days.

Understanding the effects that legalization can have on cannabis use patterns is important, as numerous studies report that youth cannabis use can have long-lasting negative impacts on the developing brain. Frequent cannabis use in adults has also been associated with an increased risk of experiencing delusions and other psychotic symptoms.

Recent studies into legalization and cannabis use patterns have generally returned mixed results.

A 2021 study focusing on students in California reported significant increases in lifetime and post 30-day cannabis use rates across nearly all demographic groups following the state’s legalization of recreational cannabis. Similarly, a nationwide study carried out earlier this year also found that underage cannabis use had become more common following the passage of state-level legalization bills.

In contrast, a systematic review and meta-analysis published in BMJ Open concluded that cannabis decriminalization and medical legalization do not result in higher levels of cannabis use among youths. Legalization for recreational purposes appeared “to possibly result in a small increase” in cannabis use. But given that only a relatively small proportion of studies could be considered to be at low risk of bias, the review authors say that further research is still required.

One study that claimed to find no association between legalization and adolescent cannabis use was later retracted after concerns were raised over how two data sets were used in the study. However, after correction and re-publication, the study authors still found there to be no link between recreational cannabis laws and adolescent use.


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