John Kapoor, the founder of US drug maker Insys, was sentenced to five and a half years in prison on Thursday for helping fuel America's deadly opioid crisis.

The 76-year-old became the first owner of a pharmaceutical company to go to prison over the crisis that has killed tens of thousands of Americans.

Kapoor was convicted in May of an elaborate scheme that saw Insys pay doctors money to prescribe the company's addictive painkiller Subsys to patients who didn't need it.

He was found guilty of criminal conspiracy, along with four other former executives of the Arizona-based firm, following a 10-week trial in Boston.

In 2017, US resident Donald Trump declared the opioid crisis a national public health emergency.

Almost 400,000 people have died from overdoses involving prescription or illicit opioids over the past two decades, according to the Centers for Disease Control and Prevention.

Nearly 48,000 people died in 2017 alone from opiate overdose.

To increase sales of their fentanyl spray Subsys, a painkiller 50 to 100 times stronger than morphine, Insys executives set up a system of large-scale bribes.

Between 2012 and 2015, they paid health professionals to prescribe large quantities of the highly addictive drug, which was approved to alleviate severe pain in terminal cancer patients.

Doctors were encouraged to recommend the spray to patients who did not need it and also in excessive doses.

Officially, the bribes were paid as fees to doctors speaking at seminars for health professionals to praise the benefits of the drug.

The company's aggressive marketing tactics reportedly also included sales representatives making a rap video to promote the drug.

In some years, Insys paid more than $10 million in bribes in this way. The heads of the firm also set up a scheme to mislead health insurance companies to reimburse patients for the costs of the drug.

According to Insys' annual report, Subsys sales reached $329.5 million in 2015.

The group went public in 2013. Kapoor resigned from the board in October 2017, a few days after his indictment.