“As our awareness of the addiction and overdose risks associated with use of opioid painkillers such as Oxycontin and Vicodin grows, individuals with chronic pain and their medical providers may be opting to treat pain entirely or in part with medical marijuana, in states where this is legal,” Colleen L. Barry, PhD, an associate professor in the Department of Health Policy and Management at the Johns Hopkins Bloomberg School and senior author of the study, said in a statement. The analysis was conducted by researchers from the Bloomberg School and the Philadelphia Veterans Affairs Medical Center.

The unanalyzed data show that states with medical marijuana laws consistently had higher average prescription drug overdose death rates throughout the years studied, 1999 to 2010, as depicted in the chart below. But after the researchers applied their statistical analysis—accounting for the years a law was in place and comparing death rates after laws were passed in each state to those expected based on pre-passage data—they found that medical marijuana policies were associated with a 24.8 percent lower annual rate of painkiller overdose deaths. In absolute terms, those states had 1,729 fewer overdose deaths in 2010 than would be predicted by trends in states without such laws.

To conduct their analysis, published in the peer-reviewed JAMA Internal Medicine on Monday, the researchers relied on CDC death certificate data. The rate of overdose deaths rose in all states over the study period. California, Oregon and Washington were the only three states with medical marijuana laws in place before 1999, while ten more joined by 2010.

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While the results seem to suggest that the availability of marijuana may lure people away from prescription drugs, the researchers caution against drawing that conclusion too decisively. Factors specific to each state, such as attitudes about health, could explain the association between marijuana laws and overdose deaths, for example. They did, however, explore whether other policies targeting painkiller abuse—such as having a drug monitoring program or allowing pharmacists to ask for patient ID before dispensing medication—were associated with declines in overdose death rates and did not find a significant connection. Among the study’s shortcomings is the fact that the researchers could not account socioeconomic, racial, ethnic, medical or psychiatric differences among state populations.