A billionaire drug company boss has been found guilty of bribing doctors to prescribe a highly addictive and dangerous painkiller, in the first criminal conviction of a pharmaceuticals chief executive over the US opioid epidemic.

A jury in Boston found John Kapoor, the founder of Insys, and four colleagues guilty of defrauding insurance companies by misleading them into paying for a spray form of fentanyl, a drug that is 100 times more potent than morphine and 50 times stronger than heroin.

Mr Kapoor directed a scheme to bribe doctors to prescribe the spray, branded as Subsys, to patients that did not need it. Prosecutors said that helped to fuel the opioid epidemic and cost lives.

“These patients were used. Their pain was exploited,” US attorney Nathaniel Yeager told the court. “The decisions, the money, the strategy came from the top.”

Mr Kapoor’s company paid doctors more than $1m speak at sham events ostensibly promoting Subsys. He now faces up to 20 years in prison.

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Fentanyl is only approved in the US as a medication to alleviate cancer pain, but Insys marketed it via a network of doctors to a much wider and more lucrative market.

Mr Kapoor’s lawyers acknowledged that Insys paid doctors, but argued that he believed this was part of a legitimate discussion about the benefits of fentanyl spray.

As part of its sales tactics, Insys produced a rap video for its staff describing ways to boost sales featuring a large bottle of Subsys including the line: “I got new patients, and I got a lot of ’em.”

During the trial, witnesses testified that an Insys regional sales rep who had worked as a stripper gave a lap dance to a doctor in a nightclub as part of efforts to persuade him to prescribe Subsys.

Aggressive marketing helped to rapidly boost sales of Subsys to $500m in 2017, from $14m five years earlier.

Drug manufacturers, distributors and pharmaceutical firms are facing hundreds of lawsuits over the costs of a crisis estimated to have killed more than 400,000 people over two decades.

Mr Kapoor and his co-defendants – former Insys executives and managers Michael Gurry, Richard Simon, Sunrise Lee and Joseph Rowan – have denied any wrongdoing and indicated that they planned to appeal.

“Dr Kapoor is disappointed in the verdict, as are we,” said Beth Wilkinson, Mr Kapoor’s lead attorney. “Four weeks of jury deliberations confirm that this was far from an open-and-shut case.”

Prosecutors said the case was part of the US government’s effort to prosecute those deemed responsible for fuelling a nationwide epidemic of addiction to prescription painkillers.