The Minnesota attorney general has joined dozens of other states in filing a lawsuit against pharmaceutical companies accused of manipulating the market for a medication used to treat opioid addiction.

Minnesota Attorney General Lori Swanson joined 35 other state attorneys on Thursday in filing a lawsuit in U. S. District Court against the makers of Suboxone, which is used to minimize opioid withdrawal symptoms. The lawsuit alleges that the drug’s manufacturer, Indivior PLC, unlawfully extended its patent on the drug to prevent competition from generic drugs.

Minnesota Attorney General Lori Swanson said that the company used a technique called “product hopping,” which is illegal under state and federal antitrust laws.

“A brand-name pharmaceutical company will try to obstruct generic competitors from coming in, keep their own monopoly profits for themselves on a patented drug by tweaking slightly a drug in non-material ways,” Swanson said. “Basically, keeping a monopoly on the drug and being able to keep charging monopoly prices by making this tweak.”

Suboxone originally was manufactured in pill form. But while the medication was still protected by patent, Indivior created and marketed a dissolvable film version of Suboxone that extended the patent. The lawsuit alleges that the drug company used marketing and price differences between the two forms of the drug to lower demand for the tablet version of the medication.

The lawsuit alleges that the drug company also expressed unfounded safety concerns about tablets in an attempt to delay U.S. Food and Drug Administration approval of generic versions of Suboxone.

Opioid overdose deaths, whether caused by prescription opioid painkillers or heroin, now account for about 30,000 deaths each year in the United States. Suboxone has become a popular treatment to minimize withdrawal symptoms, with over $1 billion in sales each year, according to the suit.

Swanson said the lawsuit’s goals are to open the market up to more generic competition, to get an injunction against the company for its approach, and to get money back for customers.

Pharmaceutical company MonoSol Rx, which specializes in converting medications from tablet form to film is also named in the lawsuit. Neither company named in the lawsuit immediately responded to a request for comment.