Thirty-five U.S. states and the District of Columbia filed a lawsuit on Thursday against opioid drug maker Indivior alleging it tried to keep generic versions of a drug off the market.

The British company, which was spun off from Reckitt Benckiser in 2014, is the maker of Suboxone, a drug used to treat patients addicted to heroin and other painkillers.

The lawsuit by the states alleged that Indivior took steps to get patients to switch to a dissolvable oral strip version of Suboxone.

Indivior said in a statement that it would continue to “vigorously defend” its position.

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A tablet version of the drug was first approved for sale in the United States in 2002, and although it did not have patent protection, the Food and Drug Administration gave the company seven-year “orphan drug” treatment so it could recover research and development costs.

The lawsuit charged that when Indivior’s exclusive right to sell the drug neared its expiration in 2009, it took steps to block generic versions from entering the market and that another company, MonoSol Rx, conspired with Indivior to achieve that goal.

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According to the lawsuit by the states, Indivior told the FDA that it planned to offer a new “sublingual film,” or dissolvable strip version of the drug, which had been licensed to Indivior by MonoSol Rx.

Since the strip was not equivalent to the tablet version of the drug, it would prevent pharmacists from prescribing a cheaper generic alternative to patients.

The lawsuit claims that the company persuaded the FDA to approve the strip by raising concerns that the tablet posed high exposure risks to children. It later used those pediatric safety claims again to try to persuade the FDA not to approve generic tablets of Suboxone.

The FDA rejected the company’s arguments, but by the time the generics hit the marketplace, many patients had already switched over to using the dissolvable strip, the complaint alleged. There is no generic version of the dissolvable strip.

“My office will not permit drug companies to engage in anticompetitive conduct that unlawfully extends their monopolies — and their monopoly profits — on drugs,” New York state Attorney General Eric Schneiderman said in a statement.