In a joint action, 35 US states and the District of Columbia (which governs the country’s capital city of Washington) have filed an antitrust lawsuit against British drugmaker Indivior, alleging the company tried to keep cheaper generic versions of Suboxone off the market. The popular medication, worth $2 billion in annual sales, is prescribed for treatment of addiction to heroin and painkillers.

Along with Indivior – formerly part of Reckitt Benckiser – the suit filed in the US district court for the eastern district of the state of Pennsylvania charges that pharmaceutical dissolving-film producer MonoSol Rx, based in New Jersey, was a co-conspirator in keeping the market under wraps.

In 2002, while still trading as Reckitt-Benckiser, the UK drugmaker was granted exclusive rights to sell Suboxone tablets for a seven-period – the medication had been awarded orphan drug status. When the rights expired in 2009, the suit alleges, the company worked with MonoSol Rx to make a sublingual strip form that the two companies subsequently marketed as being safer than tablets and thus posing fewer risks for children.

The partners then are said to have petitioned the US Food and Drug Administration to impose strict packaging requirements on potential competitors while trying to switch patients to their own product. In the view of the states’ attorneys general, this practice amounts to illegal “product hopping,” making small changes to a product to keep less expensive products off the market.

Without the product hop, a majority of Invidior’s patients would have been switched to generics, New York’s attorney general, Eric Schneiderman, said in statement. Schneiderman, who in 2014 took aim at drugmaker Actavis for discontinuing Namenda IR, a drug that was losing patent protection, earlier this month charged that Mylan Pharmaceuticals had unfairly limited competition for its emergency allergy treatment EpiPen.

In the Invidior case, Schneiderman also suggested that the FDA was in part complicit. In approving the new version, he said, the agency “acknowledged that the film version may actually present more of a threat to children in the case of accidental exposure.”

Indivior has said it will “continue to vigorously defend its position.”

In the US, where drug prices are the world’s highest, efforts to keep prices down through increasing the share of generics are being thwarted by pharmaceutical producers’ “life-cycle management” schemes such as product-hopping, a study published in August by the Journal of the American Medical Association said.