April 7 (Reuters) ― A physician ally of Purdue Pharma LP whose views helped drive the explosive growth in the use of addictive pain relievers for common aches and pains in the United States has agreed to testify against the OxyContin maker and other drug companies, newly disclosed court records show.

Dr. Russell Portenoy, a professor at the Albert Einstein College of Medicine, was an early advocate for the use of opioids for chronic pain, a position he shared in medical journal articles, with regulators, at physician conferences and in other forums. He also was named as a defendant in some of the lawsuits filed by cities, counties and states seeking to hold opioid makers ― including Endo and Mallinckrodt Plc ― and their distributors liable for the cost of the U.S. opioid epidemic.

But Portenoy began talking to plaintiffs’ lawyers as early as January 2018 and later struck a deal with the plaintiffs to serve as a cooperating witness, the records show. In exchange for his dismissal from the suits, Portenoy provided the plaintiffs with documentation of opioid makers’ payments to him over the years, as well as a 36-page declaration that lays out what he would say on the witness stand.

Portenoy’s previously confidential cooperation agreement and declaration were made public Friday as a part of a discovery ruling by David Cohen, a special master in the federal court in Cleveland, Ohio, where hundreds of the opioid lawsuits have been consolidated. In an email, Cohen said it was possible the records should have been filed under seal out of public view.

Their disclosure offers a glimpse into the largely secretive evidence gathering phase of the litigation. Purdue executives have said the privately-held company is considering bankruptcy in the face of its potential liability. The company avoided having to stand as a defendant in a trial scheduled for May in Oklahoma by agreeing to pay the state $270 million.

Portenoy could not be reached for comment. His lawyer did not immediately respond to an email seeking comment.

Purdue spokesman Bob Josephson declined to comment on Portenoy’s declaration and new role in the litigation.

Other companies, including Endo and Mallinckrodt, did not immediately respond to requests for comment.

Portenoy, who has held leadership positions at Memorial Sloan-Kettering Cancer Center and Mount Sinai Beth Israel, was among the first physicians to espouse opioids as an option for the treatment of chronic pain, a condition that afflicts tens of millions of Americans, in medical journal articles published in the 1980s and 1990s. Later, he shared his views with physicians at conferences and in training videos sponsored by opioid makers, he said in the declaration.

Previously, physicians had been trained to reserve opioids for cancer and end-of-life pain. Opioids are chemically similar to heroin and the risk of addiction was viewed as too great for their widespread use.

In his declaration, Portenoy said he never altered his positions because of opioid makers’ payments for research, speeches, consulting and other work. But, he said, the companies paid only for work that supported their interests and cited it selectively “to promote opioids by referencing the positive statements that I made repeatedly without providing the background, analysis of the literature, and cautions that accompanied these positive statements.”

These “unbalanced communications” encouraged the prescribing of opioids to patients unsuited for them and by physicians who lacked the skills to manage addiction, abuse and overdose, Portenoy said. All of this, he concluded, “contributed to the rising incidence of drug addiction and overdoses.”

Opioids are involved in about 50,000 deaths a year in the United States. In an effort to staunch the toll, the U.S. Centers for Disease Control and Prevention in 2016 recommended against opioids as a first-line treatment for chronic pain.

In a tentative decision issued on Friday, special master Cohen ruled that Portenoy may serve as a witness for the plaintiffs - just not in the first trial scheduled to take place in federal court in Cleveland later this year. Cohen was responding to a complaint from the defendant companies that they had inadequate notice of Portenoy’s new role.

Josephson said Purdue believed the special master’s ruling was appropriate. He did not elaborate.

(Reporting by Lisa Girion in Los Angeles and Maria Caspani in New York; Editing by Diane Craft and Daniel Wallis)