Concord — A new state report confirms what advocates have long suspected — that private insurance companies paid drug treatment providers substantially less than Medicare rates for the same common services.

“It has been a historic issue where providers in this field are paid less,” said Michele Merritt of the advocacy group New Futures. “You can’t expand treatment capacity as a provider if you are being chronically underpaid by an insurance carrier.”

The New Hampshire Insurance Department report comes in response to the state’s ongoing opioid crisis, which led to more than 420 drug overdose deaths last year. It looked at data from Aetna, Anthem, Cigna, Harvard Pilgrim and United Healthcare between October 2014 and September 2015. The report found all five carriers, to varying degrees, paid less than Medicare rates for common substance abuse services in New Hampshire.

Cigna had the highest reimbursement rate compared with the market average, but the insurer still fell 17 percent shy of Medicare rate reimbursements. United Healthcare fared the worst, the report found. The insurer reimbursed treatment providers 33 percent beneath Medicare rates.

Generally, commercial insurance companies pay providers more than Medicare and Medicaid. Neither carrier disputed the findings.

“Cigna’s contracted service rates are based on agreements with each in-network health care provider,” spokesman Mark Slitt said.

The Insurance Department plans to follow up with a formal examination, according to Commissioner Roger Sevigny.

“This study raises some important questions,” Sevigny said in a statement, “such as whether low payment rates affect the ability of the health care system to meet the current level of demand for treatment and whether these payment rates comply with federal mental health parity requirements.”

The state’s lawmakers have sought to beef up treatment and recovery services recently, in large part by expanding insurance coverage and limiting pre-authorization requirements.

Gov. Maggie Hassan is “troubled” by the findings, her spokesman said.

Low reimbursement rates can limit treatment providers’ ability to expand patient services and attract qualified workers.

“When the private sector is paying markedly lower than what the public sector is paying in some cases, that is cause for concern,” said Tym Rourke, chairman of the Governor’s Commission on Alcohol and Other Drug Abuse Prevention, Treatment and Recovery.

More than two dozen providers in the state were included in the report’s findings. They were paid by insurers at rates that ranged from 139 percent higher than Medicare to 35 percent lower.

The discrepancy may exist because many substance abuse treatment providers don’t have experience negotiating with insurance carriers, Merritt said. Until recently, most providers received funding from state grants. Under the Affordable Care Act, all carriers must now cover substance abuse services.

For some providers, it’s still a struggle just to get paid.

“We’re still battling trying to get paid, let alone paid less,” Nashua’s Keystone Hall Vice President Annette Escalante said.

The Insurance Department is in the process of reviewing whether carriers comply with legal coverage requirements for substance abuse treatment.

The report is expected to be made public this fall.