A doctor who led the charge for mass prescribing of opioids in the US, and was then paid by Purdue Pharma to help drive sales of OxyContin, is to testify against the drugmaker and other companies facing a slew of lawsuits over America’s opioid epidemic.

Dr Russell Portenoy, who many experts believe did more than any other specialist to erode longstanding caution within the medical profession over prescribing opioids because of addiction fears, has agreed to cooperate with lawyers for cities and counties suing drugmakers, distributors and pharmacies in return for dropping legal actions against him.

In a newly released statement to an Ohio court hearing a combined lawsuit of more than 1,600 cases, Portenoy accuses drugmakers of underplaying the dangers of opioids and of pushing them on patients who did not need them. The doctor said the industry overstated the benefits of narcotics painkillers and “understated the risks of opioids, particularly the risk of abuse, addiction and overdose”.

But if he appears as a witness, Portenoy, a professor at the Albert Einstein College of Medicine in New York, will be vulnerable to defense claims he laid the ground for the drugmakers with an evangelical promotion of opioids based on massaged scientific studies he has since repudiated.

He was particularly influential in persuading primary care doctors that their fears of patients becoming addicted were unfounded. He made the claims in medical training videos as well as Purdue Pharma’s marketing material for OxyContin, the powerful opioid that kickstarted an epidemic that has claimed more more than 350,000 lives over the past two decades.

In his court declaration, Portenoy criticised opioid manufacturers for failing to respond “as evidence of increasing adverse affects mounted”.

“The opioid manufacturers should have tempered their positive messaging about opioids with a greater focus on risk, particularly as early signals of opioid risk emerged,” he said in the document.

Portenoy acknowledged he had contracts and grants with numerous pharmaceutical companies including Purdue Pharma and Endo. In his court statement he denied that drug company payments influenced his public statements on opioids. But he said industry funding of his work was directed only to areas that would help promotion of the drugs. The pain specialist said opioid makers selectively quoted his work to highlight the positives about opioids while stripping out cautionary qualifications.

Portenoy said this led to the prescribing of opioids to patients who did not need them and so “contributed to the rising incidence of drug addiction and overdoses”.

Portenoy’s part in the rise of mass prescribing of opioids goes back to the 1980s when he was at the forefront of a movement to take narcotics used to alleviate the suffering of dying cancer patients and use them to treat people living with conditions such as chronic back pain or arthritis. But first he needed to break down what he regarded as the medical profession’s unreasonable caution about prescribing opioids dating back to the early 20th century.

In 1986, Portenoy co-authored a paper in the medical journal Pain claiming that a study of cancer patients showed opioids were not the dangerously addictive drugs feared by the medical profession. The study was of only 38 patients and the results were mixed, with more than one-third failing to benefit from the drugs. It also lacked the standard scientific rigour of control groups.

But the paper had a significant impact and tapped into a frustration among a group of younger pain doctors at their inability to offer anything more than superficial relief to patients whose lives were dominated by debilitating pain.

The Pain paper marked the start of a dramatic shift in attitudes to opioids within the medical profession that was strongly pushed by Purdue Pharma and other drug companies. Portenoy toured the country, calling opioids a gift from nature and pushed access to the drugs as an ethical argument.

In 1993 he spoke to the New York Times of a “growing literature showing that these drugs can be used for a long time, with few side-effects, and that addiction and abuse are not a problem”.

Years later, Portenoy admitted his sweeping conclusions were based on “weak, weak, weak data”.

“I would cite six, seven, maybe ten different avenues of thought or avenues of evidence, none of which represented real evidence. And yet what I was trying to do was to create a narrative so that the primary care audience would look at this information in toto and feel more comfortable about opioids in a way they hadn’t before. In essence, this was education to destigmatize and because the primary goal was to destigmatize we often left evidence behind,” he admitted years later to Physicians for Responsible Opioid Prescribing.

“Clearly, if I had an inkling of what I know now then, I wouldn’t have spoken in the way that I spoke. It was clearly the wrong thing to do. ”