The nation’s largest drug distributor is being accused of illegally pooling leftover cancer medication from single-dose vials and selling it to health-care providers, who treated patients with it and often billed government programs for reimbursement.

A lawsuit unsealed Wednesday in New York seeks unspecified damages from McKesson Corp., the fifth-largest U.S. public corporation of any kind.

It says a McKesson subsidiary violated the federal False Claims Act by selling the medication, providing kickbacks by offering the pooled drugs at a discount and repackaging them under non-sterile conditions. That left the drugs open to contamination that could have harmed patients, the lawsuit claims.

A private company filed the lawsuit on behalf of itself, the United States, 31 states, New York City, Chicago and the District. The lawsuit alleges that McKesson repackaged medication from at least 2007 to 2010.

The McKesson subsidiary, US Oncology “repackaged or compounded the oncology drugs in an unlawful manner that was unapproved by the United States Food and Drug Administration and in non-sterile conditions, resulting in false claims being submitted for drugs,” according to the lawsuit.

McKesson said in a statement that “patient safety, compliance with the law and maintaining the trust of our customers are top priorities for us. While we have not yet formally received the complaint, we reject the allegations as they’ve been reported and plan to vigorously defend the company in court if this case moves forward.”

An FDA spokeswoman said the agency does not comment on pending litigation.

The federal government accused one of McKesson’s main competitors, AmerisourceBergen, of a similar scheme last year. That company eventually pleaded guilty to misdemeanor criminal charges and paid nearly $900 million in fines over related allegations involving pre-filled syringes. But it did not admit to the drug-pooling accusations.

Last year, McKesson paid $150 million, the largest fine ever by a distributor of opioid painkillers, to settle federal allegations that it had failed to report suspicious orders of narcotics by drugstores and other providers that diverted them to the black market.

The new allegations center on pre-filled syringes containing drugs for cancer and sideeffects that McKesson produced and sold to oncology centers, hospitals and physicians. The medication typically was sent from manufacturers to the company’s plant in single-dose vials. To ensure that each syringe could be properly filled, each vial contained as much as 10 percent more than would be put in a syringe.

Because the vials contain no preservatives, a single puncture leaves the remaining medication open to contamination, so the distributor is supposed to dispose of them.

But McKesson allegedly “harvested” the leftover medication, producing about one extra syringe for every 10 legitimates ones, according to George Carpinello, one of the attorneys for the plaintiffs.

In some cases, the company put false FDA identification numbers on the unlawfully produced syringes, Carpinello said. Some health-care providers billed Medicare and Medicaid for the treatment they gave patients, an action the lawsuit described as defrauding the government.

It is unclear whether patients were harmed by the alleged practice, Carpinello said. The immune systems of cancer patients are compromised, leaving them prone to infections.

But “at best, McKesson was indifferent” to the possibility of harming patients, he said.

The company encouraged purchase of the pre-filled syringes, rather than the vials, by offering them at a discount, the lawsuit contends. In September 2007, for example, a syringe full of one cancer drug cost $327.42 while a vial of the same medication cost $346.99, according to the lawsuit.

It is difficult to know how many unlawfully prepared syringes were sold, Carpinello said. From 2007 to 2010, the company shipped millions of syringes, and 95 percent of the drugs ordered by the one private company among the plaintiffs came in that form, the lawsuit shows.