WASHINGTON — Health insurance companies must cover mental illness and substance abuse just as they cover physical diseases.

The Obama administration issued new regulations Friday that spell out how a 5-year-old mental health parity law will be administered.

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Health and Human Services Secretary Kathleen Sebelius said the rule should put an end to discrimination faced by some mental health patients through higher out-of-pocket costs or stricter limits on hospital stays or visits to the doctor.

The law, signed by President George W. Bush, was designed to prevent that. But mental health advocates said health insurers at times sidestepped lawmakers’ intentions by delaying requests for care and putting in place other bureaucratic hurdles. They described the new Obama administration rule as necessary to ensure patients get benefits they are entitled to receive.

The administration pledged to issue a final mental health parity rule as part of an effort to reduce gun violence. Officials said they have now completed or made significant progress on 23 executive actions announced in response to the school massacre in Newtown, Conn., last December.

The 2008 mental health parity law affects large group plans. It does not require they offer mental health coverage, but if they do, that coverage must be equal to what is provided for patients with physical illnesses. Meanwhile, the Affordable Care Act extends the parity protections for those participating in individual and small group health insurance plans.

“For way too long, the health care system has openly discriminated against Americans with behavioral health problems,” Sebelius said.

Sebelius said access to mental health coverage had already been improving since passage of the 2008 mental health parity law. She said larger employer health insurance plans have eliminated higher cost-sharing for inpatient mental health care and most plans have done the same for outpatient care.