Illinois Attorney General Lisa Madigan filed an antitrust lawsuit Friday against Reckitt Benckiser Pharmaceutical, now known as Indivior, alleging that they attempted to alter one of their drugs in a scheme to block generic competitors.

The suit alleges that the pharmaceutical company conspired with MonoSol Rx to change Suboxone from a tablet to a dissolvable film in order to prevent or delay generic alternatives and therefore maintain monopoly profits, according to a statement from Madigan’s office. Suboxone is used to treat heroin and other opioid addictions by easing cravings.

No generic alternatives to Suboxone are currently available, Madigan’s office said.

When Reckitt introduced the drug in 2002, they had exclusivity protection on the tablet for the first seven years, meaning no generics could enter the market during that time, prosecutors said. Before that period ended, Reckitt worked with MonoSol Rx to create the dissolvable film version.

Over time, Reckitt allegedly steered the market away from the tablet to the film through marketing, price adjustments and other methods, according to Madigan’s office. Once the majority of prescriptions were written for the film, they removed the tablet from the U.S. market.

Madigan, and the 35 other attorneys general who filed the suit, allege that these actions were illegal “product hopping,” where a company slightly changes a product to extend patent protections.

The film version of Suboxone was no better than the tablet, and Reckitt continued to sell the tablet in other countries after removing from the U.S. market, prosecutors said. They also allegedly expressed unfounded safety concerns about the tablet version and intentionally delayed FDA approval of generic versions.

As a result, consumers and purchasers have paid artificially high monopoly prices since Reckitt’s exclusivity protection on Suboxone ended in 2009, Madigan’s office said. Annual sales of the drug have topped $1 billion during that time.

The suit accuses Reckitt and MonoSol Rx of one count of conspiracy to monopolize and one count of illegal restraint of trade, which violates the Sherman Act and state laws, prosecutors said.

Madigan and the 35 other attorneys ask the court to stop the companies’ anticompetitive conduct, to restore competition and to order appropriate relief for consumers and the states, Madigan’s office said.

“These companies rigged a system to ensure they profited at the expense of the people who depended on this drug to treat and recover from addiction,” Madigan said in the statement.