By Lauren Smith

Updated: 22 May 2023 & medically reviewed by Dr. Kimberly Langdon

State legalization of recreational cannabis doesn’t increase the use of illicit drugs or lead to other negative psychosocial outcomes among adults, a new study has found, throwing into question the decades-long portrayal of the substance as a “gateway drug.”

Laboratory worker holds up cannabis bud for inspection

Results based on longitudinal twin studies

A study, published in the January issue of the journal Psychosocial Medicine, drew on data from two of the country’s largest longitudinal twin studies: one overseen by CU Boulder’s Institute for Behavioral Genetics (IBG) and the other housed at the Minnesota Center for Twin Family research.[1] The participants, now between ages 24 and 49, have been followed since adolescence.

The changing drug policies of Colorado and Minnesota provided researchers an opportunity to compare the effect of cannabis legalization on the cohorts.

In 2014, Colorado became one of the first states to legalize recreational marijuana, while the substance remains illegal for recreational purposes in Minnesota (although permitted for medicinal use). 

A previous study using the twin databases found that residents of states where cannabis is legal for recreational purposes use cannabis around 20% more than those living in states where it’s illegal.[2]

Researchers now wanted to know if increased cannabis use leads to psychosocial harm, including the use of other illicit drugs.

To get a general sense of the impact of legalization, researchers compared the 40% of twins who live in states with legal recreational cannabis to those residing in states where it’s illegal.

Additionally, they identified 240 pairs of twins in which one lives in a state with legal cannabis and one doesn’t and compared the siblings to each other.

“This co-twin design automatically controls for a wide range of variables, including age, social background, early home life and even genetic inheritance,” said John Hewitt, professor of psychology and neuroscience at CU Boulder. “If the association holds up, it provides strong evidence that the environment, in this case legalization, is having an impact.”[3]

23 measures of psychosocial distress examined

To assess the twins, researchers used 23 measures of “psychosocial distress,” including the use of illicit drugs such as cocaine and heroin, psychotic behavior, cognitive problems, financial distress, unemployment, and relationship difficulties.

Overall, they found little difference between the psychosocial outcomes in states with legal cannabis and states without. 

Legalisation didn’t appear to increase the risk of cannabis use disorder. It also didn’t increase the use of other illicit drugs.

“For low-level cannabis use, which was the majority of users, in adults, legalization does not appear to increase the risk of substance use disorders,” said study co-author Dr. Christian Hopfer, professor of psychiatry at IBG and CU Anschutz who studies substance abuse disorders.

One difference researchers did find was in alcohol-related harms. Twins in states where cannabis is legal showed fewer symptoms of alcohol use disorder and were less likely to engage in risky drinking behaviors, such as driving while intoxicated.

But in general, the researchers found no evidence that legalizing recreational cannabis improves psychosocial outcomes.

“No drug is risk-free”

Around half of Americans now live in states where recreational cannabis is legal and other states, including Minnesota, are mulling legalization.

Opponents and proponents of cannabis legalization frequently cite potential outcomes of the shifting policies as their rationales. But with legalization under a decade old, there has been little scientific evidence about its long-term effects so far.

This latest study could undercut the worst fears about cannabis legalization, including the conventional wisdom that it’s a gateway drug to harder substances.

“We really didn’t find any support for a lot of the harms people worry about with legalization,” said lead author Stephanie Zellers, who began the research as a graduate student at CU Boulder’s IGB. “From a public health perspective, these results are reassuring.”

However, the study also doesn’t provide conclusive evidence that cannabis is entirely safe for everyone. Specifically, it didn’t look at the impact of legalization on adolescents or distinguish between the kinds and dosages of cannabis used.

“Our study suggests that we should not be overly concerned about everyday adult use in a legalized environment, but no drug is risk-free,” said Hewitt. “It would be a mistake to dismiss the risks from higher doses of a drug that is relatively safe in small amounts.”

But the study may help redirect the debate about the public health impacts of cannabis legalization to questions of vulnerability.

“I would love for us to get past this question of, ‘Is legalization good or bad?’ and move toward more specific questions like, ‘Who is most at risk? Who can benefit the most? And how?’ So that people can make informed choices,” Zellers said. 

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