The cost of prescription painkillers has been gradually decreasing for patients, with a growing share of the costs now covered by insurers, according to new research.

{mosads}That shift over the last decade could be partly responsible for the dramatic increase in the use of the powerful, and increasingly deadly, drugs, according to a study published Monday in the journal Health Affairs.

The average out-of-pocket price for 100 milligrams of morphine plummeted to 90 cents in 2012, down from $4.40 in 2001.

Out-of-pocket spending on prescription painkillers made up about 53 percent of the total $2.3 billion market in 1999.

By 2012, that figure was just 18 percent, though the total amount of money spent on painkillers tripled to about $7.4 billion.

Over the same period, deaths from overdoses in the U.S. nearly quadrupled.  

Both private insurers and public programs like Medicare are now covering an increasing share of the costs of opioids.

Medicare Part D, the prescription drug program founded in 2006, has spent double the amount on opioid drugs for its small group of beneficiaries under age 65 than it has for its entire over-65 population.

People under 65 used $1.8 billion worth of opioids painkillers, compared to $637 million for those over age 65, the study found.

Medicare and Medicaid are also taking up a far greater share of national spending on opioids.

In 1999, the programs covered 9 percent of the costs. By 2012, that figure jumped to 35 percent of all spending.  

The study is the first of its kind to examine who is footing the bill for opioids, which researchers said could be a first step in addressing the national crisis.  

The opioid epidemic has become a top political agenda for Congress in 2016. The Senate passed sweeping, bipartisan legislation earlier this year, though it includes no new funding, and the House is expected to take up a similar bill next week. 

“Understanding the source of payment for opioid pain relievers may help identify potential ways to prevent overdose, because payers have a financial incentive to reduce inappropriate prescribing,” the researchers wrote.