Expanding Medicaid rolls under the Affordable Care Act may have saved as many as 8,132 people from fatal opioid overdoses, virtually all involving heroin and fentantyl, a study released Friday suggests.

The research is the latest evidence that allowing more people to enroll in Medicaid has saved lives and improved health.

The researchers concluded that additional access to drug-abuse treatment was linked to a 6% lower overdose rate for states that allowed more people to enroll in Medicaid than in states that did not. That translated into 1,678 to 8,132 fewer deaths in those states from 2015 to 2017, they wrote in an examination of data from 49 states and the District of Columbia.

The research, published in JAMA Network Open, was not designed to prove cause and effect. Rather, it found an association between the decline in overdose deaths in the 32 states and the District of Columbia that had expanded Medicaid at the time of the study.

Other reviews of Medicaid expansion have shown that low-income people with asthma and diabetes were less likely to be admitted to hospitals; that smokers received help quitting tobacco; and that some states sharply increased spending on services for people with substance use disorders.

Critics of President Donald Trump’s efforts to undermine the Affordable Care Act, also known as Obamacare, have frequently said that doing so conflicts with his work to stem the nation’s opioid crisis, which has taken more than 400,000 lives over the past two decades.

The new study appears to be the first to conduct a county-by-county examination of overdose deaths and Medicaid expansion. It found an 11% decline in the death rate from heroin overdoses and a 10% drop in deaths from illegal fentanyl and its analogs.

“As states invest more resources in addressing the opioid overdose epidemic, attention should be paid to the role that Medicaid expansion can play in reducing opioid overdose mortality,” most likely through the provision of anti-addiction medication, the researchers wrote.

The study found no link between Medicaid expansion and declines in overdose deaths from prescription narcotics. In an interview, the leaders of the research team–Nicole Kravitz-Wirtz of the University of California, Davis School of Medicine and Magdalena Cerda, of the NYU Grossman School of Medicine –said that Medicaid expansion likely brought many of the poorest Americans into the insurance program. That group was more likely to be using street drugs than prescription narcotics, they said.

Other research has indicated that by dramatically increasing the availability of the overdose antidote naloxone on the streets, Medicaid expansion also may have cut the number of overdose deaths.

The study unexpectedly found a rise in overdose deaths caused by methadone in states that expanded Medicaid. The drug is most commonly known as a treatment for opioid use disorder, but it is also prescribed for pain, though much less frequently than drugs like oxycodone.

Kravitz-Wirtz said that Medicaid recipients are more likely to receive the drug as a painkiller, and that further research is needed to explain why the review found the increase in those deaths. Previous research has shown methadone to be safe and effective in suppressing the use of opioids, particularly heroin.