By Ronnie Cohen

(Reuters Health) - Physicians wrote significantly fewer prescriptions for painkillers and other medications for elderly and disabled patients who had legal access to medical marijuana, a new study finds.

In fact, Medicare saved more than $165 million in 2013 on prescription drugs in the District of Columbia and 17 states that allowed cannabis to be used as medicine, researchers calculated. If every state in the nation legalized medical marijuana, the study forecast that the federal program would save more than $468 million a year on pharmaceuticals for disabled Americans and those 65 and older.

No health insurance, including Medicare, will reimburse for the cost of marijuana. Although medical cannabis is legal today in 25 states and the District of Columbia, federal law continues to prohibit its prescription in all circumstances.

The new study, published July 6 in Health Affairs, was the first to ask if there's any evidence that medical marijuana is being used as medicine, said senior author W. David Bradford in a phone interview. The answer is yes, said Bradford, a health economist and a professor at the University of Georgia in Athens.

"When states turned on medical marijuana laws, we did see a rather substantial turn away from FDA-approved medicine," he said.

Researchers analyzed Medicare data from 2010 through 2013 for drugs approved by the U.S. Food and Drug Administration (FDA) to treat nine ailments - from pain to depression and nausea - for which marijuana might be an alternative remedy.

They expected to see fewer prescriptions for FDA-approved drugs that might treat the same conditions as cannabis. Indeed, except for glaucoma, doctors wrote fewer prescriptions for all nine ailments after medical marijuana laws took effect, the study found.

The number of Medicare prescriptions significantly dropped for drugs that treat pain, depression, anxiety, nausea, psychoses, seizures and sleep disorders.

For pain, the annual number of daily doses prescribed per physician fell by more than 11 percent.

"The results show that marijuana might be beneficial with diverting people away from opioids," Bradford said.

A 2014 study (bit.ly/1pYZf8d) found that opioid overdose death rates were on average nearly 25 percent lower in states where medical marijuana was legal compared to states where it remained illegal. Chronic or severe pain is considered a primary indicator for medical marijuana in most states where it is legal.

Nearly two million Americans either abused or were dependent on prescription opioids in 2014, according to the U.S. Centers for Disease Control and Prevention (CDC). Since 1999, more than 165,000 Americans have died from prescription opioid overdoses.

Addiction psychiatrist Dr. Kevin Hill questioned whether medical marijuana patients might in some cases be getting inferior or incorrect treatment, and if so, whether the resulting extra healthcare costs would overshadow the Medicare drug savings. Hill, a professor at Harvard Medical School in Boston, was not involved in the new study.

"Fewer opioid prescriptions in medical marijuana states might be a good thing, but I am concerned about the overall quality of care delivered in medical marijuana specialty clinics," he told Reuters Health in an email.

He criticized the implementation of medical marijuana laws in many states as often leading to "medical care that is of poor quality."

Part of the problem stems from a dearth of research into the efficacy of medical marijuana.

Although California became the first state to legalize medical marijuana in 1996, federal law enacted by Congress in 1970 continues to put cannabis in the same category as heroin, Schedule 1 of the Comprehensive Drug Abuse Prevention and Control Act, and finds it has no medicinal value. Consequently, research has been severely limited.

Sheigla Murphy, a medical sociologist who was not involved in the current study, praised it as a major contribution to the literature on the role of medical marijuana in older adults.

Murphy directs the Center for Substance Abuse Studies in San Francisco and has done prior research on marijuana and baby boomers. She said some older adults prefer marijuana to painkillers and sleeping pills.

"It fits with the problems of older age, problems with sleeping, depression, arthritis, worn-out body parts that begin to hurt. Marijuana can relieve these without the side effects of grogginess and worrying about addiction," she said.

"As we're trying to reduce the number of pain medications, I think marijuana would be a welcome addition to the pharmacopeia," she said. "The one thing we know is no one has ever died of it."