Legalizing recreational cannabis at the state level does not increase substance use disorders or use of other illicit drugs among adults and, in fact, may reduce alcohol-related problems, according to new CU Boulder research.

The study of more than 4,000 twins from Colorado and Minnesota also found no link between cannabis legalization and increases in cognitive, psychological, social, relationship or financial problems.

"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 Institute for Behavioral Genetics (IBG). "From a public health perspective, these results are reassuring."

For the study, published Jan. 5 in the journal Psychological Medicine, researchers at the University of Minnesota, CU Boulder and the CU Anschutz Medical Campus tapped data from two of the nation's largest and longest-running twin studies: one housed at IBG and the other at the Minnesota Center for Twin Family Research.

Researchers have followed participants, now aged 24 to 49, since adolescence, collecting data on use of alcohol, tobacco, cannabis (aka "marijuana"), and several illicit drugs, as well as measures of "psychosocial health."

In 2014, Colorado became one of the first two states in the U.S. to commence legal sales of recreational marijuana. In Minnesota, recreational use of cannabis remains illegal.

The power of twin studies

By comparing the 40% of twins who live in recreationally legal states to those who live in states where it is still illegal, the researchers set out to get a general sense of the impact of legalization.

In addition, by specifically comparing twin to twin in 240 pairs in which one lives in a state that has legalized and the other in a state that has not, researchers sought to determine what changes, if any, cannabis legalization causes.

"This co-twin design automatically controls for a wide range of variables, including age, social background, early home life and even genetic inheritance" that can influence health outcomes, 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."

In a previous study, the group found that identical twins living in states where cannabis is legal used it about 20% more frequently than their twin in states where it's illegal.

The logical next question: Does more use mean more problems?

To find out, the team compared survey results looking at 23 measures of "psychosocial distress," including use of alcohol and illicit drugs such as cocaine and heroin, psychotic behavior, financial distress, cognitive problems, unemployment, and relationships at work and at home.

"We included everything we had data on with the goal of getting a well-rounded look at the impacts on the whole person," said Zellers. "Big picture, there's not much there."

Surprisingly, they found no relationship between legalizing cannabis and heightened risk of cannabis use disorder, or cannabis addiction.

While many critics of legalization have expressed concern that cannabis could serve as a "gateway drug" to other more harmful substances, the researchers found no changes in illicit drug use after legalization.

"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 co-author Dr. Christian Hopfer, a medical doctor and professor of psychiatry at IBG and CU Anschutz who studies substance abuse disorders.

Twins in states where cannabis is legal did show fewer symptoms of alcohol use disorder: Specifically, they were less likely to engage in risky behavior after drinking, such as driving while intoxicated.

'No drug is risk-free'

The authors caution that the study did not look at impacts on adolescents and did not look closely at the kinds and dosages of cannabis people were using.

"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."

The study found no evidence that legalizing cannabis benefits psychosocial health, either.

Today, 21 states have legalized recreational cannabis. Half the U.S. population lives in a place where it is legal, and several more—including Minnesota—are considering legalization.

Zellers, who recently earned a doctorate from the University of Minnesota, hopes the paper, and more to come, will help inform policy conversations about legalization.

"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.