The case of Insys Therapeutics has played out for several years as suspicions first cropped up in 2014 that the company aggressively marketed its powerful opioid painkiller Subsys, often for off-label uses. Now, the feds have charged five New York-area physicians for taking bribes from the drugmaker in exchange for writing more scripts.

In addition to the new complaint, authorities announced that two former Insys employees have taken guilty pleas and are cooperating with the government. New York doctors Gordon Freedman, Jeffrey Goldstein, Todd Schlifstein, Dialecti Voudouris and Alexandru Burducea face up to 20 years in prison for their alleged participation in the scheme, according to a Justice Department release.

"Payments from pharmaceutical companies should not influence how doctors prescribe—especially when a potent and dangerous drug like fentanyl is involved," Manhattan U.S. attorney Geoffrey S. Berman said in a statement. He added that the doctors violated a "solemn oath" and used "their patients as an instrument for profit."

The doctors pleaded not guilty and posted bail, according to Reuters.

Allegations against Insys and its former management have been piling up in recent years, and in October, authorities made their way to the company's billionaire founder John Kapoor, charging him with racketeering and other felonies. Kapoor responded by saying "I am confident that I have committed no crimes and believe I will be fully vindicated after trial."

RELATED: Feds charge Insys' billionaire founder John Kapoor in Subsys probe

Before Kapoor, six other Insys executives and other lower-level employees have faced charges relating to the alleged bribes-for-scripts arrangement. Prosecutors say the company set up a "speakers bureau" to recruit doctors to write more Subsys scripts, holding "sham" speaking events that didn't offer any educational elements. Sign-in sheets were routinely forged, according to the government.

And so, prosecutors allege, the company used the events to funnel money to doctors in exchange for more Subsys scripts, even though many of the prescriptions were outside of the drug's FDA label: to treat breakthrough pain from cancer. One patient's parents sued over "reckless" marketing that they said caused their daughter's death. RELATED: Blockbuster sales of Insys painkiller raise suspicions of off-label marketing

In one trip to New York, Insys executives allegedly took two of the newly charged doctors and others to a Manhattan strip club and spent $4,100. Schlifstein's scripts increased dramatically after the trip, according to the complaint.

In 2014, a New York Times investigation into higher-than-expected Subsys sales raised suspicions about the situation. Meanwhile, Insys has overhauled its management and is working to improve its image. The company has said it could have to spend $150 million on a potential settlement.