A subsidiary of pharmaceutical firm Indivior on Friday admitted to a felony charge as part of a $600 million settlement after the US Justice Department accused it of pushing a treatment for opioid addicts unlawfully.

"When a drug manufacturer claims to be part of a solution for opioid addicts, we expect honesty and candour to government officials, as well as to the physicians and patients making important treatment decisions based on those representations," Deputy Assistant Attorney General Michael Granston of the Justice Department's civil division said in a statement.

The settlement, which requires approval from a federal judge, comes after a grand jury last year said that Indivior marketed Suboxone Film, an opioid-based drug, as a safe and controllable treatment for opioid and heroin addiction.

The grand jury indictment said that Indivior -- until 2014 known as Reckitt Benckiser Group -- sought to boost sales by exaggerating the safety of the drug and connecting patients with doctors who were known to overprescribe it and other opioids, taking advantage of the country's huge opioid addiction crisis.

Beginning in 2010, the indictment said, Indivior illegally told health care providers and programs that Suboxone Film, which contains the opioid buprenorphine, was better and safer than similar drugs, when in fact it was not.

The aim was to force generic competitors from the market, the indictment said.

Reckitt Benckiser Group agreed in July 2019 to pay $1.4 billion -- the largest opioid-related settlement ever -- to resolve its liability in the case.

On Friday, Indivior Solutions, a subsidiary of the parent company based in Slough, Britain and Richmond, Virginia, pled guilty to one count of making false statement relating to health care matters.

The $600 million fine will be paid along with its parent companies, the Justice Department said.

Indivior Chief Executive Officer Mark Crossley said in a statement the incident "does not reflect the values Indivior has strived to demonstrate and uphold during our long history of partnering with health care providers, policymakers, and communities to fight the opioid crisis."