Marijuana legalization advocates have long argued that pharmaceutical companies, which could lose out if marijuana is legally available, are some of the staunchest supporters of pot prohibition, and now an Arizona company is making their case for them.

According to campaign finance reports posted online by the Arizona secretary of state's office, fentanyl manufacturer Insys Therapuetics has donated $500,000 to foes of the Prop 205 marijuana legalization initiative.

Fentanyl is a synthetic opioid several dozen times more potent than heroin. It has been linked to numerous opioid overdose deaths across the country, especially when mixed with heroin. Marijuana, on the other hand, has no reported overdose deaths, ever.

Insys isn't just any pharmaceutical company. Its sole product is Subsys, a sublingual fentanyl spray, and the company has shown that it's willing to bend the rules to sell that product. In the past month alone, two former company employees pleaded not guilty to federal charges related to an alleged kickback scheme to get doctors to prescribe Subsys, and Illinois Attorney General Lisa Madigan filed a lawsuit against the company charging that Insys hawked the drug to doctors for off-label prescribing.

Insys' "desire for increased profits led it to disregard patients' health and push addictive opioids for non-FDA approved purposes," Madigan wrote.

While Subsys is the only product the company currently markets, it says on its website that it is also working "to develop pharmaceutical cannabinoids." It's not much of a leap to wonder whether the company is backing the continued criminalization of marijuana users in order to protect potential market share for its products in development, and legalization supporters were quick to do so.

Responding to a query from US News & World Report, the anti-legalization group Arizonans for Responsible Drug Policy said it would not return the donation. Instead, it released a statement expressing gratitude for the donation and pointing out that Insys is an Arizona-based company, unlike the Marijuana Policy Project (MPP), which backs the legalization effort.

The MPP-backed Arizona Campaign to Regulate Marijuana Like Alcohol responded with a statement from campaign director J.P. Holyoak, who laid into both Insys and the opposition group that took its money. He said:

We are truly shocked by our opponents’ decision to keep a donation from what appears to be one of the more unscrupulous members of Big Pharma. You have a company using profits from the sale of what has been called ‘the most potent and dangerous opioid on the market’ to prevent adults from using a far less harmful substance. In addition to selling an extremely potent and dangerous opioid, they have been under investigation by numerous states and the federal government for the manner in which they have done so.

"Their homepage touts their development of ‘pharmaceutical cannabinoids,’ which are synthetic versions of chemical compounds found in marijuana," he continued. "It appears they are trying to kill a non-pharmaceutical market for marijuana in order to line their own pockets."

"Our opponents have made a conscious decision to associate with this company. They are now funding their campaign with profits from the sale of opioids—and maybe even the improper sale of opioids. We hope that every Arizonan understands that Arizonans for Responsible Drug Policy is now a complete misnomer. Their entire campaign is tainted by this money. Any time an ad airs against Prop. 205, the voters should know that it was paid for by highly suspect Big Pharma actors," he concluded.