Medicaid would cover 15 days of inpatient addiction treatment per month under a new proposed plan by the Centers for Medicare and Medicaid Services, which runs the program. Inpatient rehab would be covered for anyone enrolled in a Medicaid managed care plan.

Until now, people on Medicaid generally have had to rely on funds from state and local sources for drug or alcohol treatment, NPR reports.

Some treatment providers say the plan doesn’t go far enough. “Where they came up with the 15 days, I don’t know, but it’s not based on research,” said Mike Harle, head of the nonprofit treatment program Gaudenzia, which serves about 20,000 patients a year in Pennsylvania, Maryland and Delaware. He said 15 days is not enough to achieve a positive outcome. “Do you know how expensive that would be, with no outcome?” he said.

Pennsylvania uses a provision in federal law to receive federal reimbursement for longer rehab stays for some patients, the article notes. State officials say it is likely they will no longer be able to use that provision if the new proposal is enacted.

The National Institute on Drug Abuse notes in its guide to drug addiction treatment that there is no predetermined length of treatment. “However, research has shown unequivocally that good outcomes are contingent on adequate treatment length,” the guide states.

There is little good data about the optimal length of an inpatient stay for addiction treatment, according to Dr. Jeffrey Samet, a professor at Boston University’s Clinical Addiction research unit. Because of the lack of data, private insurance plans vary widely in terms of how many days of inpatient treatment they cover, he noted.

Cindy Mann, a former administrator at the Centers for Medicare and Medicaid Services, says 15 days is a good start, considering Medicaid has not funded any residential treatment until now.