MONDAY, Aug. 25, 2014 (HealthDay News) -- States that have legalized medical marijuana tend to experience an unexpected benefit -- fewer overdose deaths from narcotic painkillers, a new study suggests.

Access to medical marijuana is associated with 25 percent fewer prescription drug overdose deaths each year compared to states where medical pot is illegal, according to findings published Aug. 25 in JAMA Internal Medicine.

What's more, states that pass medical marijuana laws see their overdose death rates decrease dramatically in the years immediately afterward, researchers reported.

The study authors believe that people suffering from chronic pain tend to rely on medical marijuana when they have that option, which reduces the risk of addiction and overdose that accompanies use of narcotic medications.

"We think that people with chronic pain may be choosing to treat their pain with marijuana rather than with prescription painkillers, in states where this is legal," said lead author Dr. Marcus Bachhuber, a researcher with the Center for Health Equity Research and Promotion at the Philadelphia Veterans Affairs Medical Center.

But some experts caution against drawing conclusions from the study.

"I don't know what to make of the paper. I'd be very, very careful saying that medical marijuana laws decrease risk of opiate [narcotic] overdose," said Dr. Bradley Flansbaum, a hospitalist at Lenox Hill Hospital in New York City. "It's a very loose association."

The study used data from the U.S. Centers for Disease Control and Prevention to determine the prescription painkiller overdose death rate for each state between 1999 and 2010, and then took into account whether and when each state had passed a medical marijuana law.

Overdose deaths from prescription painkillers have skyrocketed over the past two decades, increasing 118 percent between 1999 and 2011, according to the CDC.

The agency estimates that every day 113 people die from drug overdoses in the United States, and another 6,700 land in the emergency room from an overdose.

Currently, 23 states allow medical marijuana to ease chronic pain and other conditions.

The researchers reviewed death certificates from all states between 1999 and 2010, when 13 states had legalized marijuana.