Furthermore, from 1999 to 2010 (the period of time we studied), states implemented various measures in response to the threat of opioid painkiller overdoses, including central registries of controlled substance prescriptions, laws allowing pharmacists to request identification before filling a prescription and laws increasing oversight of pain management clinics. These measures, too, might affect rates of opioid painkiller deaths, regardless of the legality of medical marijuana.

We designed our study to allow us to compare state-level rates of opioid painkiller overdose deaths before and after the passage of medical marijuana laws, while controlling for these and other concurrent state and national trends.

Using death certificates compiled by the Centers for Disease Control and Prevention, we found that the rate of prescription painkiller overdose deaths increased in all states from 1999 to 2010. But we also found that implementation of a medical marijuana law was associated with a 25 percent lower yearly rate of opioid painkiller overdose deaths, on average. In absolute terms, we estimated that states with a medical marijuana law had a total of about 1,700 fewer opioid painkiller overdose deaths in 2010 than would be expected based on trends before the laws were passed.

This is the first study that we know of to suggest that medical marijuana laws could contribute to a decline in drug overdose deaths, and therefore it should be read with caution. Our study was not a controlled experiment, and it is possible that states with and without medical marijuana laws differed over time in important ways that we did not or cannot measure and that could explain, at least in part, our results.

However, if medical marijuana laws are in fact reducing opioid overdose deaths, the next step is to figure out how and why. That people are replacing opioid painkillers in part or entirely with medical marijuana for chronic pain treatment is one possibility. Another possibility is that the availability of medical marijuana is changing the behavior of people who are addicted to and abuse or misuse opioids. We know that marijuana and opioids stimulate a common receptor in the brain’s reward pathways, but we don’t know whether people who misuse or abuse opioids for recreational purposes would switch to marijuana in states where it is legal for medical purposes.