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  • A new study finds legalizing recreational cannabis does not increase substance misuse of cannabis or other illicit drugs.
  • Researchers compared the 40% of twins living in a state with recreational cannabis policies to those living in states where it is still illegal.
  • The researchers caution that their study included only adults 24 years or older, adding that cannabis legalization may have a different impact on younger adults.

State legalization of recreational cannabis does not increase problematic use of cannabis or other illicit drugs among adults, a new study suggests.

It may even reduce alcohol-related problems, although researchers caution that their findings related to alcohol are “difficult to interpret” and require additional study.

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

For the study, published Jan. 5 in the journal Psychological Medicine, researchers examined data from two long-running studies of twins maintained by the Minnesota Center for Twin Family Research and the Colorado Center for Antisocial Drug Dependence.

The study included over 4,000 twins, first assessed in adolescence and now aged 24 to 49. During the final assessment, researchers collected data on participants’ use of alcohol, cannabis, tobacco and several illicit drugs.

Researchers also measured their “psychosocial health,” including personality disorders, financial distress, legal problems, unemployment, workplace behavior and cognition.

To examine the impact of recreational cannabis legalization, researchers compared the 40% of twins living in a state with recreational cannabis policies to those living in states where it is still illegal.

In addition, they compared twin to twin in the 240 pairs where one lived in a state with recreational cannabis and the other in a state without.

This smaller twin-to-twin analysis allowed researchers to control for a range of factors that might affect a person’s substance use or other health outcomes.

This is because twins are the same age and have a similar social background, home life, parental attitudes toward alcohol and other drugs, etc. Identical twins also share the same genes, while fraternal twins share half of their genes.

Magdalena Cerdá, DrPH, a professor and director of the Center for Opioid Epidemiology and Policy at NYU Grossman School of Medicine in New York City, said one strength of the study is its inclusion of twins.

This allowed researchers to “account for potential individual level differences between people living in states that legalized recreational cannabis use versus those that did not,” she said.

However, “there are still remaining differences between states that chose to legalize cannabis compared to states that did not,” she said, which could potentially have impacted the results.

In a previous study, the same group of researchers found that twins living in a state with recreational cannabis policies used cannabis about 20% more often than their twin living in a state where it is illegal.

The new study showed a similar increase in the frequency of cannabis use in states that allow recreational cannabis.

However, researchers found that twins living in a state with legal recreational cannabis did not have an increased risk of cannabis use disorder, or problematic cannabis use.

Cerdá said the increase in cannabis use after legalization is consistent with other studies, including a 2020 study by her and colleagues, published in JAMA Psychiatry.

Unlike the new study, her study showed a small increase in cannabis use disorder among 12- to 17-year-olds and adults 26 years or older.

Cerdá pointed out that the new study does show somewhat of an increase in cannabis use disorder when researchers compared twins to twins, but it’s not statistically significant.

This lack of significance, she said, might be due to the small number of twin pairs living in states with differing cannabis policies.

The authors of the new study also found no link between recreational cannabis legalization and increased use of other illicit drugs, or impact on psychosocial health.

In addition, twins living in states with legal recreational cannabis showed fewer symptoms of alcohol use disorder, the results showed.

Cerdá said other research on the effect of cannabis legalization on alcohol use is inconsistent, with some studies finding a decline in alcohol use, while others showing an increase.

However, she questions the new study’s finding of a decrease in symptoms of alcohol use disorder.

If there had been a real change in alcohol use disorder, “you would have also expected to see a decrease in the quantity of alcohol use,” she said, “and they did not find that.”

The new study only showed a decrease in risky behavior while drinking, such as driving while intoxicated. The amount of alcohol consumed was not affected by cannabis legalization.

The researchers caution that their study included only adults 24 years or older, adding that cannabis legalization may have a different impact on adolescents or younger adults.

In addition, they did not have data on the kinds and dosages of cannabis that people were using.

Some research has found that daily use of cannabis or use of high-potency products may increase the risk for symptoms of psychosis.

Dr. O. Trent Hall, an addiction medicine specialist at The Ohio State University Wexner Medical Center in Columbus, said while the results of the new study are reassuring, they are far from the final word in the debate over cannabis legalization.

”Cannabis legalization is still relatively new,” he said. “Some harms from cannabis might not be obvious until more time has passed.”

In addition, there are some positive societal effects of legalization that weren’t measured by this study, he said, such a reduction in the number of people who go to jail for minor cannabis-related offenses.

”Many people have traumatic experiences while incarcerated that may haunt them for the rest of their lives” he said. “They may also have difficulty obtaining housing and employment upon release from incarceration.”

Hall also pointed out that while population-level studies like this one can provide insight into the broad effects of cannabis legalization, the results may not apply to every person.

“At the individual level, people may still experience harm from cannabis use,” he said.