California passed the first medical cannabis law in 1996.
The bill will allow medical marijuana patients under 21 who suffer from PTSD legal access to marijuana for the first time.
The researchers analyzed data from 118,497 adults included in three national surveys: 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.
In addition, the percentage of people with "marijuana use disorders" - people who use the drug in unhealthy ways, or abuse it - has also increased at a higher rate in these states, according to the study.
The study notes some limitations. "There is no morality in keeping medicine from sick and dying patients who can benefit from medical cannabis".
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. Please see the article for additional information, including other authors, author contributions and affiliations, financial disclosures, funding and support, etc. 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. With so many states invested in the marijuana industry, it's unclear whether or not these threats will amount to anything but the fact that marijuana is still technically illegal on a federal level, has made lawmakers uneasy about triggering a crackdown.
"While research continues to gather evidence to that end, clinicians are faced with the reality reinforced by the findings from Hasin et al. that cannabis use is increasing among adults living in states that have legalized medical marijuana", they wrote.
"When you are evaluating the effects of a state law on a sample that is not state-representative, it can be misleading", Pacula said.
The study was funded by the National Institute on Drug Abuse and the National Institute on Alcohol Abuse and Alcoholism, part of the National Institutes of Health.
In other words, the rates of illegal marijuana use increased more quickly in states with medical marijuana laws, the researchers said.
"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.
"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.
In particular, the biannual poll conducted by the Colorado Department of Public Health and Environment showed that teen marijuana use fell dramatically in Colorado after the state opened its recreational marijuana market.
The nuances of state medical marijuana programs need to be studied so policymakers can balance potential harms and benefits, they said.