Illicit cannabis use and cannabis use disorders increased at a greater rate in states that passed medical marijuana laws than in other states, according to new research at Columbia University’s Mailman School of Public Health and Columbia University Medical Center.

The findings were published online in JAMA Psychiatry journal.

Laws and attitudes regarding cannabis have changed over the last 20 years. In 1991, no Americans lived in states with medical marijuana laws, while in 2012, more than one–third lived in states with medical marijuana laws, and fewer view cannabis use as entailing any risks.

The new study is among the first to analyze the differences in cannabis use and cannabis use disorders before and after states passed medical marijuana laws, as well as differentiate between earlier and more recent periods and additionally examine selected states separately.

The researchers used data from three national surveys collected from 118,497 adults: the 1991–1992 National Longitudinal Alcohol Epidemiologic Survey, the 2001–2002 National Epidemiologic Survey on Alcohol and Related Conditions and the 2012–2013 National Epidemiologic Survey on Alcohol and Related Conditions–III.

Overall, between 1991–1992 and 2012–2013, illicit cannabis use increased significantly more in states that passed medical marijuana laws than in other states, as did cannabis use disorders. In particular, between 2001–2002 and 2012–2013, increases in use ranged from 3.5 percentage points in states with no medical marijuana laws to 7.0 percentage points in Colorado. Rates of increase in the prevalence of cannabis use disorder followed similar patterns.

“Medical marijuana laws may benefit some with medical problems. However, changing state laws – medical or recreational – may also have adverse public health consequences, including cannabis use disorders,” said author Deborah Hasin, PhD, associate professor of Epidemiology at the Mailman School of Public Health and in the Department of Psychiatry at Columbia University Medical Center. “A prudent interpretation of our results is that professionals and the public should be educated on risks of cannabis use and benefits of treatment, and prevention/intervention services for cannabis disorders should be provided.”

While illicit use of marijuana decreased and marijuana use disorder changed little between 1991–1992 and 2001–2002, both use and disorder rates increased between 2001–2002 and 2012–2013. In 1991–1992, predicted prevalences of use and disorder were higher in California than other states with early–medical marijuana laws (use: 7.6 percent vs. 4.5 percent; disorder: 2 percent vs. 1.15 percent). However, the predicted prevalence of past year use in California did not differ significantly from states that passed laws more recently. In contrast, the prevalences of use and disorder increased in the other 5 states with early medical marijuana laws.

“Future studies are needed to investigate mechanisms by which increased cannabis use is associated with medical marijuana laws, including increased perceived safety, availability, and generally permissive attitudes,” Dr. Hasin also noted.