In states where medical marijuana is legal, the rates of deaths caused by painkiller overdoses are lower than in states where medical marijuana is illegal, a new study has found.

In the study, researchers examined the rates of deaths caused by opioid painkiller overdoses between 1999 and 2010, using death-certificate data compiled by the Centers for Disease Control and Prevention (CDC). Opioid painkillers, such as OxyContin and Vicodin, are often prescribed for people with chronic pain. 

In line with previous findings, the new results showed that the rates of opioid overdose deaths increased in all states over the study period. However, the 13 states that had laws allowing the use of marijuana for medical reasons during that time had, on average, death rates from opioid overdoses that were about 25 percent lower compared with the rates in states without these laws.

In 2010, these lower rates translated to about 1,700 fewer deaths than what would have been expected if medical marijuana had not been allowed in these states, the researchers said. [5 Surprising Facts About Pain]

The new findings are rather striking, said Marie Hayes, a professor of psychology at the University of Maine, who wrote a commentary about the study.

"This is the first time that we have population-level data that suggests that marijuana has medical benefits," Hayes said.

The researchers also found that the link between lower rates of opioid-overdose deaths and medical-marijuana laws strengthened over time. The number of deaths was reduced by nearly 20 percent in the first year after a state's law was implemented, and was about 34 percent lower five years after the law's implementation, according to the study, published today (Aug. 25) in the journal JAMA Internal Medicine.

In the United States, death rates from drug overdoses have more than tripled since 1990, according to the CDC. Nearly three out of four prescription-drug overdoses are caused by opioid painkillers. In 2012, U.S. health care providers wrote 259 million prescriptions for opioid painkillers, enough for every adult in the country to have a bottle of the pills, according to the CDC.

Opioid painkillers are particularly dangerous because users may develop tolerance to the drug over time, and may seek higher doses to experience the same effects, experts say. Mixing these medications with other drugs and alcohol is also dangerous and may result in overdose. About half of prescription-painkiller deaths involve at least one other drug, such as cocaine or heroin, according to the CDC.

It is not clear how, exactly, medical marijuana is related to lower opioid overdose rates. But the protective effect of medical marijuana laws seems convincing, Hayes said.

"You can often find very suggestive correlations that turn out to be not true, and are controlled by other variables," Hayes told Live Science. "But in this case, by looking at the number of years following implementation [of the law], that means that rather than just having a snapshot of the correlation at some time, they were able to actually find a continuation of the trend over years."

The researchers also looked at other potential factors that could be linked to the decline in overdose death rates — for example, whether the states had prescription-monitoring programs or oversaw pain-management clinics that prescribe these medications. "It turns out that those factors had no effect or very little effect," Hayes said.

A safer alternative?

Although more studies are needed to explain the effects of medical-marijuana laws, the researchers have some ideas as to what may be happening in the states that have legalized the substance for medical use.

For instance, it is possible that patients may use medical marijuana, instead of prescription painkillers, to treat their chronic pain, the researchers said. Some studies have suggested that marijuana may, indeed, provide pain relief for some people, the researchers said.

It is also possible that people who are already taking opioids for chronic pain may supplement them with medical marijuana and may be able to lower their painkiller dose, which, in turn, would lower their risk of overdose, the researchers said.

However, only about 60 percent of people who died from an opioid overdose had legitimate prescriptions for these medications, according to the study. The remaining 40 percent of the people who died and didn't have an opioid prescription were likely not patients with pain, and may have had other addictions as well, Hayes said. Increased access to marijuana may offer an alternative for these people, she said.

This study alone will not be sufficient to recommend wide adoption of medical-marijuana policies, but it does provide evidence to support the legalization of medical marijuana, Hayes said.  

If future studies confirm that medical-marijuana laws play a role in lowering opioid-overdose deaths, such laws and policies could be used to reduce people's risk of using opioid painkillers, the researchers said.

Email Bahar Gholipour or follow her @alterwired. Follow Live Science @livescience, Facebook & Google+. Originally published on LiveScience.

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This story was originally written for LiveScience and was republished with permission here. Copyright 2014 LiveScience, a TechMediaNetwork company.