Obamacare’s Medicaid expansion, which gave millions of low-income adults access to health insurance, was linked to a 6 percent reduction in opioid overdose death rates — potentially preventing thousands of deaths — according to a new study in JAMA Network Open.

The study looked at what happened in counties in states that expanded Medicaid under the Affordable Care Act by 2017, compared to counties in states that didn’t expand Medicaid, accounting for variables like demographic and policy differences. The Medicaid expansion was made optional in a 2012 Supreme Court ruling, and only 32 states and Washington, DC, had opted to expand by the study period (with the total rising to 37 in the past few years).

The study helps put to rest claims by some Republican lawmakers, particularly Sen. Ron Johnson (WI), that the Medicaid expansion made the opioid crisis worse by expanding access to painkillers. The new study, echoing others before it, suggests the Medicaid expansion had the opposite effect, and that there wasn’t a link between the expansion and more deaths caused by painkillers, with the possible — and relatively uncommon — exception of methadone used in pain treatment.

The researchers found that Medicaid expansion counties had a 6 percent lower rate in opioid overdose deaths than non-expansion counties. That was mostly due to an 11 percent lower rate of deaths involving heroin and a 10 percent lower rate for deaths linked to synthetic opioids excluding methadone (primarily fentanyl, now the biggest contributor to fatal drug overdoses in the US).

“What I find most exciting about this is we see a reduction in overdoses involving heroin and synthetics,” Magdalena Cerdá, one of the study’s authors, told me. “We haven’t been able to find a lot of policy solutions to do that, but this is one of the first policy measures that does seem to have an effect in terms of reducing overdose deaths from those opioids that are the leading contributors to overdose deaths.”

There was no association between the Medicaid expansion and deaths involving natural and semi-synthetic opioids, which includes traditional painkillers like OxyContin and Vicodin.

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There was, however, a link between the Medicaid expansion and an 11 percent increase in overdose deaths involving methadone. Although methadone is used very effectively in addiction treatment, the great majority of deaths linked to methadone, based on multiple investigations into the topic, are a result of methadone used in pain treatment. That seems to be what’s going on here: Medicaid plans have historically favored methadone for chronic pain treatment — because it’s so cheap — so perhaps Medicaid led to an increase in access to methadone for pain treatment and, as a result, more misuse and overdoses.

“This does not in any way detract from the notion that methadone is an effective way to treat opioid use disorder and reduce future overdose deaths,” Cerdá said, citing evidence that methadone reduces the mortality rate among opioid addiction patients by half or more and keeps people in treatment better than non-medication approaches. “Really, the likely explanation here is around methadone used to treat pain.”

Still, methadone is only involved in a fraction of opioid overdose deaths. On net, the study found the Medicaid expansion likely prevented fatal opioid overdoses: “[O]ur findings suggest that these states would have had between 83,906 and 90,360 deaths in the absence of the expansion, implying that Medicaid expansion may have prevented between 1,678 and 8,132 deaths in these states during those years.”

That leads to a nuanced story: The Medicaid expansion may have contributed to some more deaths in pain treatment — although there’s no empirical evidence this is true for traditional painkillers, as this study and another one found — but still ended up saving far more lives by expanding access to addiction treatment.

Other research backs this up. A 2019 study published in Health Affairs found that after West Virginia expanded Medicaid, the number of people diagnosed for opioid use disorder under the program rose — and the number of people on buprenorphine, one of the gold-standard medications for opioid addiction, increased as well. In other words, the Medicaid expansion appeared to expand access to evidence-based addiction treatment.

The study has some weaknesses. For one, it’s the first study looking at the effects of the Medicaid expansion on overdose deaths at the county level. Given that, studies using different models and assumptions may come to different conclusions. More research is needed to verify the findings.

The data used in the study for overdose deaths generally undercounts opioid overdose deaths because the drugs behind some overdoses aren’t always identified. The study also measured the effects of the Medicaid expansion on the general population, not just the beneficiaries that directly benefited from Medicaid. (A study exclusively looking at Medicaid beneficiaries could find even better results for the expansion.) And the study treated the expansion as a binary — whether states enacted the expansion or not — which blurs differences between states’ Medicaid programs that could produce different effects.

For Cerdá, those caveats are room for more research to tease out how Medicaid may bring down overdose deaths. She also wants to study the increase in methadone deaths, which could help policymakers figure out how to mitigate any risks of the expansion while keeping the benefits.

For now, though, the study suggests that state policymakers have a policy lever available to them to potentially fight the ongoing opioid epidemic, which has contributed to more than 700,000 opioid overdose deaths since 1999. As part of Vox’s Rehab Racket project, I have heard time and time again how important insurance is to obtaining quality addiction care. Lawmakers in the 13 states that haven’t moved to expand Medicaid now have a window to ameliorate at least part of the problem of poor insurance coverage leading to more cases of untreated addiction and overdose deaths.