Over the period the researchers studied, the opioid epidemic was worsening and many states — led mostly by Democrats, but sometimes by Republicans — chose to expand coverage under Medicaid, the joint federal and state health insurance program for poor people. The Affordable Care Act gave states the option of covering many more adults through Medicaid , which covers a disproportionate share of people with opioid addiction, starting in 2014.

The law also vastly expanded access to addiction treatment by designating it as an “essential benefit” that must be covered through the Obamacare marketplaces and expanded Medicaid . Buprenorphine is one of three medications that the Food and Drug Administration has approved to treat opioid addiction; there is substantial evidence that it sharply reduces the risk of dying from an overdose.

Hundreds of thousands of Americans have died from opioid overdoses over the past decade, including nearly 48,000 last year, and most people with opioid addiction are not getting treatment, according to government studies.

The study found that, on average, the rate of Medicaid-covered prescriptions for buprenorphine was much lower in states that did not expand the program; most of those states are in the South and Great Plains. In all, 33 states and the District of Columbia have expanded Medicaid since 2014 and 14 have not; they are largely led by Republicans who oppose it on partisan, ideological and fiscal grounds. In three other states — Idaho, Nebraska and Utah — voters approved ballot measures last fall directing their legislatures to expand Medicaid, but they have not yet done so.

All five of the states with the highest buprenorphine prescribing rates for Medicaid recipients — Vermont, West Virginia, Kentucky, Montana and Ohio — expanded Medicaid. West Virginia, Kentucky and Ohio also have among the highest overdose rates in the nation, but overdose deaths fell in all three states last year.