(Reuters Health) - Where medical marijuana is legal, adults are more likely to use the drug illegally and are at an increased risk of cannabis use disorder, according to a new study.
Researchers found that illegal use of marijuana and rates of cannabis use disorder rose to a greater extent in U.S. states that adopted laws legalizing marijuana for medical purposes than in states that didn't adopt such laws.
The risks of medical marijuana laws are important to convey, said lead author Deborah Hasin, of Columbia University’s Mailman School of Public Health in New York.
"Just as the case for alcohol, not everybody who uses it is harmed but there are some risks," Hasin told Reuters Health.
Previous research that focused on adolescents' use of the drug in the wake of medical marijuana laws didn't find an increase in that age group, she said.
"The laws may not be too relevant and salient to teenagers so we thought it was important to look at adults," said Hasin.
The new conclusions, published in JAMA Psychiatry, are drawn from three national studies conducted between 1991 and 2013. The studies compared illicit drug use in the past year and rates of cannabis use disorder between states with and without medical marijuana laws.
Illicit use was generally defined as illegal use of marijuana without a prescription at least 12 times during the past year. Cannabis use disorder occurs when users' lives are impaired by cravings or withdrawal, for example.
In states that never legalized medical marijuana, the average prevalence of illicit use was 4.54 percent in 1991 and 1992, rising to 6.70 percent in 2012 and 2013. By comparison, in states where the drug did become legal for medical use, the rate of illicit use went from 5.55 percent to 9.15 percent.
The pattern was similar for drug use disorder. The average rate went from 1.35 percent in 1991-1992 to 2.30 percent in 2012-2013 in states that didn't enact medical marijuana laws, and from 1.48 percent to 3.10 percent in states that did.
States that adopted medical marijuana laws had a 1.43 percentage point increase in illicit cannabis use and a 0.66 percentage point increase in cannabis use disorder between 1991 and 2013, compared to states that didn't enact those laws.
If the U.S. population remained constant over that time period, the researchers calculate, medical marijuana laws could be linked to 1.1 million additional cannabis users and 500,000 extra people with cannabis use disorder nationwide.
The new study can't say medical marijuana laws caused the increases, but Hassin said it's possible adults interpreted the laws to mean marijuana is harmless. Also, she said, marijuana dispensaries proliferated along with advertising.
"I think all of these things can contribute to increases, but we can’t pinpoint them for sure because it’s not what the study was designed to address," said Hassin.
The increase in use disorders may be partially explained by an increase in the potency of cannabis products, said Beatriz Carlini, of the Alcohol and Drug Abuse Institute at the University of Washington in Seattle.
"We are seeing (the increased potency) in states with no medical marijuana laws," said Carlini, who was not involved with the new research. "Now that you have marijuana sold at the retail level, you see that increase in the potency, too."
People should look at the larger picture when evaluating marijuana laws and policies, she suggested.
For example, she said, research has tied medical marijuana laws to fewer opioid overdose deaths. And public insurance payments for certain drugs were found to fall in states with medical marijuana laws.
In an editorial accompanying the new study, Dr. Wilson Compton of the National Institute on Drug Abuse in Bethesda, Maryland, and colleagues say policymakers need to understand which parts of medical marijuana laws are tied with positive and negative effects.
"In the meantime, it is clear that a robust system of education, prevention, and treatment is needed to minimize the negative consequences that might arise if cannabis use continues to increase," they write.