Medical marijuana
Medical marijuana could help reduce the growing number of deaths attributed to prescription drugs Getty Images

In states where it is legal to use medical marijuana to manage chronic pain and other conditions, the annual number of deaths from prescription drug overdose is 25% lower than in states where medical marijuana remains illegal, new research suggests.

Researchers from the Johns Hopkins Bloomberg School of Public Health and the Philadelphia Veterans Affairs Medical Center suggest that while medical marijuana laws can be controversial and opponents have raised concerns that they may promote cannabis use among children, they may have unintended benefits.

The findings suggest that it is possible that the wider availability of medical marijuana for people in pain might help to reduce the growing number of overdose deaths attributed to prescription pain pills.

"Prescription drug abuse and deaths due to overdose have emerged as national public health crises," said senior author Colleen Barry.

"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."

Using death certificate data compiled by the Centers for Disease Control and Prevention, the researchers found that the rate of prescription painkiller overdose deaths increased in all states from 1999 to 2010.

However, the yearly rate of opioid painkiller overdose deaths in states with medical marijuana laws was about 25% lower, on average, than the rate in states without these laws.

Three states – California, Oregon and Washington – legalised medical marijuana prior to 1999, with 10 more following between then and 2010, the time period of the analysis.

As of June 2014, another 10 states and Washington, D.C. have adopted similar laws.

"In absolute terms, states with a medical marijuana law had about 1,700 fewer opioid painkiller overdose deaths in 2010 than would be expected based on trends before the laws were passed," says the study's lead author, Marcus Bachhuber, of the University of Pennsylvania.

Yet Bachhuber cautions that the exact mechanism underlying these results is unclear.

He added that it would be attributed to people choosing alternative treatments, or medical marijuana laws might also change the way people abuse or misuse prescription pain medications, or something else entirely.

Medical marijuana laws have been passed to give access to the drug to people with chronic or severe pain, sometimes due to conditions such as cancer or multiple sclerosis. Cannabis is believed to have painkilling properties and also to relieve nausea and improve appetite.

Brendan Saloner, an assistant professor in the Department of Health Policy and Management at the Bloomberg School and a co-author of the study, said the benefits and risks of using medical marijuana to treat chronic pain remain unclear.

"Given the fast pace of policy change, more research is critical to understand how medical marijuana laws might be influencing both overdose deaths and the health trajectories of individuals suffering from chronic pain," he said.

The research was published in JAMA Internal Medicine.