Employees help customers buy recreational marijuana at a Eugene, Oregon, pot dispensary in 2015.
CNN  — 

People in US states that legalized recreational cannabis use it 20% more frequently than those in states that didn’t legalize it, a study published Thursday in the journal Addiction has suggested.

Researchers interviewed 3,421 participants who were sampled from the University of Colorado Boulder Center for Antisocial Drug Dependence and the Minnesota Center for Twin and Family Research in Minneapolis about their cannabis usage at two different points: before 2014, when it was illegal to sell recreational cannabis, and after 2014, when it became legal to sell in Colorado. Only medical cannabis was legal in Minnesota during the post-2014 portion of the study.

The participants, many of whom were born in Colorado and Minnesota but had since relocated, were surveyed pre- and post-2014 on how many days they used cannabis in the last six months, and scientists initially found there was about a 24% increase in usage in states that legalized recreational cannabis compared with ones that did not. Based on where respondents were living at the time of the surveys, nearly every state was represented, along with Washington, DC and Puerto Rico.

The study included 111 pairs of identical twins as well, with one twin living in a state that legalized recreational cannabis and the other in a state that did not legalize it. Among the identical twins, researchers found the usage increased about 20% in states that legalized recreational cannabis compared with ones that did not.

Since identical twins share so many similarities, that percentage is a more accurate estimate of the causal influence of cannabis legalization on cannabis use, said lead study author Stephanie Zellers, now a researcher at the University of Helsinki in Finland.

Identical twins share the same genes and often the same type of upbringing, both of which could influence how frequently someone uses cannabis, said Zellers, who was a doctoral candidate in psychology at the University of Minnesota at the time of the project.

“Because that 20% estimate is from the analysis controlling for measured and unmeasured variables,” Zellers said, “it is the most precise estimate of the causal influence of cannabis legalization on cannabis use.”

Dispensaries in Colorado may make it easier for people to purchase recreational cannabis.

Many states that like Colorado, have legal recreational cannabis also have an ample number of dispensaries, so it’s easy to make purchases, a factor that could have contributed to the higher usage rate, Zellers said.

No legal consequences such as fines or jail time also could affect the increased use in recreational states, she added.

“Additionally, the existence of recreational policies influences the perception of cannabis use,” Zellers said, “making it viewed as more safe and less stigmatized.”

How legalization affects usage

As more states legalize recreational cannabis use, it’s important to recognize how that legalization affects consumption, said Dr. Nora Volkow, director of the National Institute on Drug Abuse at the National Institutes of Health.

“This knowledge can be used to inform strategies for implementing legalization while minimizing the potential harms,” said Volkow, who was not involved in the study.

The statistics could also help researchers understand how recreational use affects the rate of cannabis addiction, she said.

About 30% of people who use marijuana in the United States have cannabis use disorder, the official term for marijuana addiction, according to the US Centers for Disease Control and Prevention.

In states where recreational use is legal, advertisements often label the drug as safe, Volkow said.

More research needs to be done on how safe cannabis is and its impact on the body before people jump to conclusions on safety, she said.

It’s not black and white

“People often want to say that cannabis is either all good or all bad,” Volkow said. “But in biology, nothing is black and white – there is a lot of gray.”

Frequent or prolonged cannabis use is associated with health conditions such as chronic bronchitis and schizophrenia, she said.

However, usage has also been shown to be effective at treating some pain conditions such as nausea and vomiting, Volkow said.

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People only saw mild to moderate pain relief with prescription-strength cannabis products and no benefits from over-the-counter cannabis drugs, according to a study published in June in the journal Annals of Internal Medicine.

Zellers said she hopes to conduct further research into the effects of increased cannabis use on conditions such as mental health and addiction.