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Contrary to the expectations of public health experts, the rate of frequent marijuana consumers who are diagnosed with cannabis use disorder (CUD) is not increasing, a new study reports. In fact, problematic cannabis use is actually decreasing, and that may be due to the growing number of states that have legalized access to marijuana.

According to the paper, “CUD prevalence decreased significantly across all ages reporting daily/almost daily cannabis use between 2002-2016. Cannabis dependence prevalence decreased for adolescents and young adults and was stable only among adults ages 26+ reporting daily/almost daily cannabis use.”

Recent studies have had mixed results on the prevalence of CUD—a diagnosis that includes either misuse and/or dependence—in the last two decades. Because people who consume marijuana every day or almost every day are the most at risk for problematic cannabis use, researchers at Columbia University’s Mailman School of Public Health set out to get a better understanding of this group’s general health.

Their findings were published last month in the journal Drug and Alcohol Dependence.

The study’s authors used data from the National Surveys on Drug Use and Health for the years 2002-2016. The final sample, totaling 22,651 people, included participants who were 12 and older and reported using marijuana at least 300 days in the past year.

To measure problematic marijuana use, the authors used criteria from the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, for cannabis dependence and cannabis abuse, such as:

  • Spent a great deal of time over a period of a month obtaining, using, or getting over the effects of marijuana
  • Unable to keep set limits
  • Unable to cut down cannabis use
  • Recurrent use resulting in failure to fulfill major role obligations
  • Continued use despite persistent or recurrent social or interpersonal problems

Other factors considered in the research asked whether participants had a perceived need for mental health treatment, whether a doctor had indicated they had other health issues and whether or not they’d driven under the influence of illegal drugs with and without alcohol.

Over the study period, the authors found that the prevalence of CUD decreased among their sample in all age groups: For adolescents ages 12 to 17, the rate fell by 26.8 percent; for 18- to 25-year-olds, by 29.7 percent; and for adults 26 and older, by 37.5 percent.

“Among those with past-year daily/almost daily cannabis use, there were reductions in the prevalence of DSM-IV cannabis abuse across all age groups, with reductions observed for all individual abuse items in adolescents and young adults,” the study states. “There were also reductions in the prevalence of DSM-IV cannabis dependence among adolescents and young adults, but not in adults ages 26+. Reductions in most DSM-IV dependence items were observed in young adults while reductions in only a few dependence items were found for adolescents and older adults.”

Researchers offer several possible explanations for the declining rates, many of which point to the influence of legalization. “First, the new national cannabis policy environment, with 33 states legalizing medical use and 10 states allowing recreational use of cannabis may have played a role in reducing stigma and perceptions of risk associated with cannabis use,” Silvia Martins, one of the study’s authors, said in a statement. “Secondly, increasing legalization may also be associated with changes in social attitudes resulting in fewer conflicts with relatives and friends around cannabis use.”

As a result, according to the paper, “[t]his could explain reductions in the abuse item ‘Continued use despite persistent or recurrent social or interpersonal problems,’ which reflects difficulties in interactions with others due to cannabis use.”

It’s also possible that “a sector of the population that is healthier overall” is starting to use marijuana more because of legal access, which “may have diluted the prevalence of cannabis abuse/dependence over time.” They may use “less potent” cannabis or in lower daily amounts, researchers note. Additionally, more people may feel less afraid to admit on a federal survey that they use marijuana frequently.

Ultimately, Martins said, the study’s results “contradict the predominating hypothesis that the prevalence of DSM-IV CUD would be stable, or increase, among those using with this regularity.”