Republicans confirmed Thursday that mental health and substance abuse services will not be covered under the health care plan they introduced this week.

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The legislation — which would replace former President Barack Obama's signature health care law, the Affordable Care Act (ACA) — eliminates the "essential health benefits" insurers were required to cover under Obamacare, effective 2020. Under the bill, it would be up to individual states to mandate coverage for mental health or substance abuse services through their Medicaid plans.

About 1.3 million people in the U.S. have enrolled in these treatment services since the ACA was implemented, according to a 2017 report from Harvard Medical School. An additional 2.5 million people suffering from substance abuse disorders or mental health conditions qualified for treatment after Medicaid was expanded. The idea that these groups would be put at risk of losing coverage — especially as the country continues to see record-high overdose deaths from opioids — has drug policy and mental health advocates worried.

"The GOP plan really it couldn't be worse related to coverage for mental health and substance use disorders," Gary Mendel, CEO of the group Shatterproof, which advocates policies to treat addiction, told ATTN:. "This is all politics over lives."

The reason, he said, is because —while the GOP insists its plan is more cost-effective than Obamacare — neglecting to cover these services will incur a cost of its own.

People suffering from substance abuse disorders who lose coverage will "turn to a life of crime, will turn to a life of unemployment, and there will be costs in going to hospital emergency rooms where he's not covered by insurance," Mendel said. "There will be costs in the criminal justice system. The math is there — this is an economically horrible decision, let alone the human suffering."

Shatterproof launched a lobbying campaign shortly after lawmakers started discussing their plan to repeal and replace the ACA, targeting Republicans in states that have been hardest hit by the opioid epidemic, including West Virginia, Pennsylvania, Kentucky and Ohio. Mendel said the organization plans to continue making the case for including substance abuse and mental health coverage in the GOP plan as Congress debates it.

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Some Republican lawmakers have already spoken out against the GOP's American Health Care Act (AHCA), arguing it would hurt vulnerable groups that have benefited from Medicaid expansion. In a letter to Senate Majority Leader Mitch McConnell, four Republican senators voiced their concerns:

"The Department of Health and Human Services reports that nearly one-third of individuals covered under the Medicaid expansion have a mental health or substance use disorder. As the largest payer of mental health and substance use services in the United States, it is critical that any health care replacement provide states with a stable transition period and the opportunity to gradually phase-in their populations to any new Medicaid financing structure."

The Republican divide over key provisions of the replacement plan has raised questions as to whether the bill can pass the Senate as currently written. If it does pass, though, it'd be up to each state to provide (or not) for those who stand to lose substance abuse or mental health coverage.