The second was 2001-2002, when six states allowed medical marijuana use.
The increase in illicit marijuana use and marijuana use disorders were 1.4 percent and 0.7 percent higher, respectively, in states that had legalized medical marijuana, compared with states that had no MMLs.
The researchers, led by Deborah Hasin, a Professor of Epidemiology at Columbia University, looked at data collected in three national surveys on alcohol and drug use from 1991 to 2013.
"America's real-world experience with medical marijuana regulation. finds that cannabis can be legally produced and dispensed in a responsible manner that positively affects the lives of patients, but does not inadvertently or adversely impact overall public health or public safety", Armentano said. The studies compared illicit drug use in the past year and rates of cannabis use disorder between states with and without medical marijuana laws.
That's the basic conclusion of a study published today that has reignited the debate over whether marijuana laws encourage illegal use of cannabis.
Data on people's marijuana use and rates of marijuana use disorders came from national surveys from the three time periods included, the study said.
Pacula added that people should be cautious in interpreting the new study, because it uses national survey data to evaluate state-level trends.
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.
The analysis revealed that between 1991-1992 and 2012-2013, states that had passed MMLs saw a greater increase in illicit marijuana use and marijuana use disorders than states that had not legalized medical marijuana.
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The researchers defined illegal use as any use of marijuana that didn't conform with the laws within the respondents' state.
Last November, Nevada, along with California, Maine and MA, voted to join Colorado, Washington, Alaska and OR in legalizing marijuana for recreational use.
"The early fear with the passage of these laws was that they would increase use among adolescents, but the studies are quite consistent that this did not happen", she said. The 2011 study suggests more research.
To date, 29 states in the US and the District of Columbia have legalized marijuana for medical use, and more are expected to follow. Thus, identifying factors underlying increased adult illicit use is important.
The conclusions from this latest study, however, are significantly different than other studies on marijuana laws and teen use of the drug.
The study notes some limitations.
Most recent research has concluded that marijuana legalization laws do not prompt an increase in use of the drug by teens. However, changing state laws - medical or recreational - may also have adverse public health consequences, including cannabis use disorders.
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.