Dozens of US states have accused a British multinational company of “shamelessly preying on patients in need of help” by profiteering from a drug at the forefront of combating America’s opioid epidemic.

Thirty-five states are suing Reckitt Benckiser for allegedly running “an unlawful, multi-pronged scheme” using patent laws and false safety concerns to dominate the market and maintain an artificially high price for Suboxone, a drug used to fight addiction to prescription painkillers and heroin. Federal authorities have been investigating the company over similar issues for the past four years.

In 2010, Reckitt – which manufactures well-known brands in the UK from household cleaners to Durex condoms – was fined after it admitted manipulating Britain’s National Health Service (NHS) to direct doctors away from a rival generic and toward one of its own more expensive drugs. Two years earlier a company whistleblower revealed memos that showed executives spoke about using safety and patent issues to maintain market dominance.

The legal action comes amid federal investigations and congressional hearings into the escalating costs of treatments and life-saving drugs used to combat an opioid addiction crisis that has claimed more than 250,000 lives in the US over the past 15 years.

Suboxone contains two drugs – a painkiller, buprenorphine, and an opiate blocker, naloxone – which were out of patent at the time the medicine came on to the market. But in 2002 Reckitt was granted a seven-year licence to exclusively sell Suboxone tablets because of its work devising and testing the drug.

It went on to earn Reckitt hundreds of millions of dollars a year as the opioid crisis widened and the number of patients seeking treatment for painkiller or heroin addiction rose. Government programmes such as Medicaid met the cost of treatment for some people.

The legal action by the states and the District of Columbia alleges that shortly before its Suboxone licence expired in 2009 the company conspired with a New Jersey firm, MonoSol RX, to create a new version of the drug as a film dissolved under the tongue. It has a patent until 2022.

Reckitt is accused of having incrementally increased the price of its tablets until they were 50% more expensive than the film in order to encourage patients and doctors to switch. According to the lawsuit, once most prescriptions had moved to the film, the company took the tablets off the market.

Reckitt then allegedly expressed “unfounded concerns” to the Food and Drug Administration about children getting access to Suboxone tablets in order to delay approval of generic versions of the drug. The company filed papers with the FDA claiming that the packaging for its film was more difficult for children to open than the bottles for Suboxone tablets. The safety claim was based on a single study paid for by Reckitt.

The FDA took two years to reject the move and suggested that pills were probably safer than film. But by then Reckitt had ensured that 85% of prescriptions were written for the film version of Suboxone, earning it about $1bn a year – 20% of the company’s profits.

The states taking legal action contend that this was a device to delay the sale of generic Suboxone tablets and allow Reckitt to keep the price of its film, and profits, artificially high. The company is facing questions as to why, if it believed selling Suboxone in bottles put children at risk, it did not act while it was still using them and why it continued to sell its pill version in bottles in other parts of the world.

The US’s Federal Trade Commission launched an investigation of Reckitt in 2012. Two years later, Reckitt sold off Suboxone manufacturing as a new company, Indivior.

In September, Senator Edward Markey wrote to the chair of the FTC, Edith Ramirez, requesting it to use “the full force of its authority to accelerate its investigation” into whether the company and others have “thwarted competition, unlawfully increased profits or illegally extended monopolies”. Markey also accused it of attempting to stall the investigation by withholding thousands of documents from the FTC.

Reckitt did not respond to a request for comment. Indivior, which is also named in the lawsuit, said: “We cannot comment in detail about pending litigation, but we disagree with a number of the claims made by the attorneys general, and we will contest this lawsuit vigorously.”

MonoSol Rx said the allegations “are wholly without merit and the suit is both factually and legally deficient”. It said that Suboxone film “has saved countless lives”.

It is not first time Reckitt has been accused of manipulating regulators with supposed concerns about safety and of using stalling tactics to hold up rival drugs.

In 2008, leaked emails showed Reckitt employees discussing how to delay the introduction of a generic version of Gaviscon liquid, a heartburn remedy prescribed in large quantities by the UK’s NHS. Reckitt internal documents revealed the existence of Project Eric, a secret plan by the company to manipulate doctors and regulators which a whistleblower said cost the NHS tens of millions of pounds.

The memos revealed that Reckitt executives planned what they described as “a clever idea” to switch from Gaviscon liquid to an almost identical product with a different name. In one of the memos a manager said the company could use “the rationale of health and safety” to design a slightly different product to “muddy the waters”. The new product was Gaviscon Advance, essentially the same as the liquid but with increased strength.

A senior whistleblower from the drug company told the BBC at the time: “Reckitt cheated the National Health Service. It could have saved the NHS millions of pounds. But not just the NHS – patients, doctors. They’ve cheated health professionals. I felt it had to be exposed.”

Reckitt denied doing so, saying it was “unhappy with some of the language and the sentiment … in the internal correspondence which are inappropriate” but said that the memos did not reflect its actions which were within the law.

However, in 2010, Reckitt admitted manipulating the NHS computer system to direct doctors away from prescribing cheaper versions of Gaviscon by delisting the drug after its patent expired but before the generic came on the market. The result was that when doctors searched using the brand name it failed to show the generic alternative and instead directed them to Gaviscon Advance. The company admitted breaching UK and European competition law and was fined £10m by Britain’s Office of Fair Trading.

In April 2016, Reckitt was fined by the Australian authorities over one of its painkillers, Nurofen. The company packaged the drug in different forms, claiming that each version targeted specific pains, such as migraine. It charged twice the price of the regular packets of Nurofen but they were identical products. An Australian court ordered Reckitt to pull the drugs from shops.