WASHINGTON — In an opioid epidemic that is killing tens of thousands of people nationwide, finding and paying for addiction treatment remains a challenge for low-income Americans, particularly in the South and parts of the West.

The Affordable Care Act required Medicaid, the joint state-federal health program for the poor, to start paying for all available substance abuse treatments in 2014, a provision seen as a boon for low-income people who previously were not covered for addiction treatment.

But Medicaid coverage of the most widely used opioid addiction medication, buprenorphine, varies widely among states. Many doctors don’t want to treat Medicaid patients for addiction. And red tape can make it difficult for many Medicaid recipients with addictions to get effective treatment.

All this has consequences as the nation battles the epidemic. As a group, Medicaid enrollees suffer from opioid addiction and other substance abuse disorders at a higher rate than the general population.

“What we see in far too many Medicaid plans are restrictions and approaches to addiction treatment that are not in line with the way we treat other diseases,” said Dr. Kelly Clark, president-elect of the American Society of Addiction Medicine (ASAM). “Some plans require patients to wean off of their addiction medications. Would you tell a heart patient he needs to taper off of his medication after a year?”

All states reimburse for the medication, but in many places, the fees Medicaid pays doctors are considered too low and the paperwork too time-consuming to attract an adequate number of providers willing to treat Medicaid enrollees with addictions.

In addition, coverage provisions such as prior authorizations, dosage and duration limits, requirements that patients “fail first” at cheaper treatments that do not include medication, high copays and excessive counseling requirements make it difficult for Medicaid enrollees to get effective treatment for the chronic, lifelong disease of addiction.

A new report on prescription sales across the country indicates Medicaid foots the bill for about a quarter of all buprenorphine prescriptions, but varying coverage limitations mean many low-income people who need medication-assisted treatment aren’t getting it, depending on where they live.

Vermont’s Medicaid program pays for the largest share of buprenorphine, 68 percent of all retail prescriptions in the state; Mississippi pays for the smallest share, 4 percent of prescriptions. Alabama, Arkansas, Florida, Georgia, Kansas, Louisiana, Montana, Nebraska, Tennessee, Texas, Oklahoma and Utah each pay for 10 percent or less of buprenorphine prescriptions.

Overall, U.S. pharmacies dispensed 12.5 million prescriptions for buprenorphine in the 12 months ending June 30, 2016, a 6.4 percent increase over the previous year, according to the report. Commercial insurers paid for 57 percent of prescriptions, Medicare paid for 7 percent, and patients paid out-of-pocket for 11 percent of all retail sales.

The new report — funded by Advocates for Opioid Recovery, a nonprofit group led by former U.S. Rep. Patrick Kennedy, D-R.I., former U.S. House Speaker Newt Gingrich, R-Ga., and civil rights activist Van Jones — calls on federal and state officials to adjust any Medicaid policies that may be preventing people with opioid addictions from receiving treatment with medication, which research has shown to be more effective than treatments without medication. The report was conducted by IMS Institute for Healthcare Informatics, which collects nationwide data on sales of prescription drugs.

“Even in the best of circumstances, obtaining access to opioid recovery programs that include medication is tough,” Jones said. “For the most vulnerable in our country who rely on Medicaid, this data proves it is nearly impossible in many states.”

Matt Salo, executive director of the National Association of Medicaid Directors, said an analysis of buprenorphine payments may serve as a proxy for coverage of addiction treatment in general, but it does not tell the whole story.

For Medicaid directors and governors in nearly every state, he said, the opioid epidemic “is an extraordinarily complex and multifaceted problem that involves far more than simply paying for buprenorphine prescriptions. There are no quick fixes.”

Medicaid coverage of addiction services was optional under federal law until the Affordable Care Act made it an “essential benefit” that all insurers must cover. Historically, Medicaid coverage of addiction treatment was nonexistent in many states or limited to pregnant women and adolescents.

Now, all states are required to pay for addiction treatment for all enrollees, including counseling, residential care and all three FDA-approved medications: methadone and buprenorphine, which act as replacement opioids, eliminating withdrawal symptoms and suppressing drug cravings, as well as injectable naltrexone, sold as Vivitrol, which blocks the euphoric effects of opioids and alcohol.

States were also given the opportunity to expand their Medicaid coverage to able-bodied adults, which for the first time provided coverage for millions of uninsured adults, including many who already suffered from substance abuse disorders.

The combined provisions of the Affordable Care Act represent “the biggest change in a generation for addiction services,” said Robert Morrison, executive director of the National Association of State Alcohol and Drug Abuse Directors.

But building a network of providers and developing quality standards and payment structures for addiction treatment has proved challenging for Medicaid programs. In addition, concerns over potential diversion of buprenorphine to street markets, and widespread bias against its use — which many consider trading one addiction for another — has led to a variety of coverage restrictions.

“There’s a lot of concern, some legitimate and some misguided, about the abuse and diversion of buprenorphine. Some of the rules designed to decrease misuse are really not evidence-based and often have unintended consequences,” Clark of ASAM said.