American states that allow their residents to use marijuana for medical purposes have seen a marked decline in prescription drug use among the elderly and disabled, according to a study published Wednesday.

The University of Georgia study looked at the costs of Medicare's prescription drug benefit program in 2013 — a time when 17 states and the District of Columbia had passed medical marijuana laws.

Researchers estimated the savings from lower prescription drug use in areas that had medical marijuana laws at $165.2 million US over the full year.

If every state legalized medical marijuana, the study forecast that Medicare would save more than $468 million US a year on pharmaceuticals for disabled Americans and those 65 and older.

Those figures, according to the researchers, suggest that some people are using marijuana as an alternative to prescription drugs to treat ailments such as pain, depression and sleep disorders.

"The results suggest people are really using marijuana as medicine and not just using it for recreational purposes," said study lead author Ashley Bradford in a statement.

Researchers looked at prescription data for nine conditions in which marijuana could serve as an alternative treatment to prescription drugs:

Anxiety.

Depression.

Glaucoma.

Nausea.

Pain.

Psychosis.

Seizures.

Sleep disorders.

Spasticity.

The data showed that, in the areas where medical marijuana was allowed, fewer prescriptions were written for every category except glaucoma, which was exactly what they expected. While marijuana does lower eye pressure in those with glaucoma, its effects last only about an hour. So researchers correctly predicted that the number of daily doses for glaucoma medication would increase because taking marijuana every hour would not be realistic.

"It turns out that glaucoma is one of the most Googled searches linked to marijuana, right after pain," said David Bradford, a health economist at the University of Georgia School of Public and International Affairs and co-author of the study. "No doctor is going to let [a glaucoma] patient walk out without being treated."

A medical marijuana user smells a jar of marijuana at the medical marijuana farmers market at the California Heritage Market in Los Angeles. (David McNew/Reuters) The new study, published in Health Affairs, was the first to ask if there's any evidence that medical marijuana is being used as medicine, Bradford told Reuters. The answer is yes, said Bradford. "When states turned on medical marijuana laws, we did see a rather substantial turn away from FDA-approved medicine," he said.

For pain, the annual number of daily doses prescribed per physician fell by more than 11 per cent. "The results show that marijuana might be beneficial with diverting people away from opioids," Bradford said.

Medicare does not pay for medicinal marijuana in the U.S., nor do private health insurance plans.

Figures obtained by CBC News two months ago showed that 22 per cent of the money paid to veterans by the Canadian government for prescribed drugs in the fiscal year 2015-16 went toward medical cannabis.

Kyle Atkinson, president of Trauma Healing Centers — a company that helps veterans file reimbursement claims for medicinal marijuana — told CBC News at the time that he suspected that some of the 1,700 veterans using medical cannabis may have stopped using other prescription drugs.

In 2001, Canada became the first country in the world to adopt a formal system to regulate the medicinal use of marijuana.

Tens of thousands of Canadians are now allowed to legally use medical marijuana. The Trudeau government says legislation to legalize marijuana possession will be ready by the spring of 2017.