By Dr. Jamie Zimmerman

States that have legalized medical marijuana may be reaping an unintended benefit from easing up on restrictions: They appear to have nearly 25 percent fewer deaths from overdoses involving prescription painkillers, a new study found.

The study, published today in the journal JAMA Internal Medicine, comes at a time when the United States finds itself in the throes of a growing painkiller abuse crisis. About 100 Americans die every day from narcotic painkiller overdose, according to statistics from the U.S. Centers for Disease Control and Prevention.

The researchers behind the new study suggest that because legalizing medical marijuana makes it more available to chronic pain patients, it provides a potentially less lethal alternative to pain control on a long-term basis.

Three states had medical marijuana laws prior to 1999, and an additional 10 states passed laws providing some legal access to marijuana during the study period. Today, 23 states and the District of Columbia have laws allowing access to medical marijuana.

Lead study author Dr. Marcus Bachhuber said that while he and his team expected to find differences in painkiller-associated deaths among states with different medical marijuana laws, they did not anticipate such dramatic differences.

"We [found] it surprising that the difference is so big," said Bachhuber, who is a physician and researcher at the University of Pennsylvania.

Bachhuber and his colleagues analyzed data on all 50 states from 1999 to 2010 and found that, while opioid overdose rates continued to climb across the United States, the numbers climbed much slower in states with medical marijuana laws. As a result, these states had 25 percent fewer deaths from opioid overdose.

Also, this effect increased in the years after the laws were enacted, suggesting the laws themselves may cause the difference.

"In my practice, I take care of a lot of people with chronic pain," Bachhuber said. "Sometimes, people with chronic pain would say only marijuana worked or they tried marijuana as a painkiller and found it worked better than prescription pills.

"One day, talking with colleagues, we wondered how this would work in states where marijuana is legal."

The findings lend additional weight to the idea that medical marijuana legalization may protect some patients who take it from the potentially harmful side effects of other medications, experts not involved with the research said.

Dr. Igor Grant, chair of psychiatry at the University of California-San Diego and director of the Center for Medical Cannabis Research, said one possible explanation for the link seen in the new study is known as the "opioid-sparing effect."

In other words, pain patients may benefit from combining opioid painkillers with less toxic medications that also provide pain relief.

"This isn't a new idea," Grant said. "Physicians have used combination drugs for a long time, such as acetaminophen with an opioid. By putting several different pain medications together, they are able to reduce the overall opioid dose, and thus decrease the risk of overdose."

The other side of the coin to more permissive medical marijuana laws, however, is the effect it may have on recreational use of the drug. Research has shown that legalizing medical marijuana tends to increased use among adults.

Even study author Bachhuber agreed that this is a potential problem to consider. "This study raises the possibility that there is an unintended public health benefit of medical marijuana laws, but we still need to collect more information to confirm or refute what we've found," he said.

Grant, too, said legalizing medical marijuana comes with serious considerations. But studies like these suggest that there may be unanticipated benefits as well, he said.

"Not to say cannabis is trivial and couldn't have a bad public health impact," Grant said. "But, we shouldn't jump to conclusions."