WASHINGTON — As millions of Americans gain health coverage through the Affordable Care Act’s Medicaid expansion, experts say their higher rates of mental health and substance abuse disorders will be difficult to treat due to a lack of counselors and behavioral therapists who accept Medicaid patients.

In the District of Columbia and the 25 states where the expansion is under way, nearly 1.2 million uninsured adults newly eligible for coverage will have substance abuse problems, according to federal estimates, and more than 1.2 million are projected to have some sort of mental illness. An estimated 550,000 of those will have serious mental disorders that impair their everyday functioning.

As these patients seek treatment for any number of problems, a shortage of caregivers — from physicians to dentists — will pose a major challenge for Medicaid, the federal-state health program for poor people and those with disabilities. The lack of providers may be most acute in the area of mental health services.

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The Medicaid expansion extends coverage to adults who earn up to 138 percent of the federal poverty level. Those with behavioral problems will be treated at community mental health agencies and health centers that serve low-income patients. This new expansion population will strain the limited resources of these facilities, many of which already have staff shortages and waiting lists for behavioral treatment.

Further complicating the problem: Most mental health therapists in private practice won’t treat Medicaid patients because of the program’s low reimbursement rates.

And now that the health care law has made mental health treatment a mandatory, or “essential,” benefit for millions of people with private coverage, the demand for therapists and counselors will increase, making it even harder for Medicaid’s community-based providers to recruit and retain behavioral health professionals.

“As more people are insured, more people are also going to be seeking services,” said Rusty Selix, the executive director of the California Council of Community Mental Health Agencies. “Our biggest concern is more competition for a limited number of professionals and the cost pressures that’s going to create. We’re going to have to pay more to retain people. It’s supply and demand.”

Already, the demand far outstrips the supply.

Nearly 91 million Americans live in federally designated mental-health-professional shortage areas, where there’s only one psychiatrist for at least every 30,000 residents. That’s compared with only 59.4 million who live in primary-medical-health-professional shortage areas and 46.7 million who reside in dental-health-professional shortage areas.

Fifty-five percent of U.S. counties — all of them rural — have no psychiatrists, psychologists or social workers, according to the U.S. Department of Health and Human Services.

Filling the needed positions nationally would take 1,846 psychiatrists and 5,931 other professionals, federal estimates show.

“That’s a critical problem. And the Medicaid expansion will make that even worse,” said Susan Mandel, the CEO of Pacific Clinics, the largest community mental health agency in Southern California.

Joel Miller, the executive director and CEO of the American Mental Health Counselors Association, said he was expecting to see “pretty significant increases” of 20 to 25 percent in the numbers of psychologists, mental health counselors, social workers, and marriage and family therapists over the next five years, based on student enrollment trends.

“I’m very much an optimist,” he said. “But sure, if you look at the current capacity, there are holes. There are gaps in inner cities and in rural areas.”