An embattled pharmaceutical company that sells the powerful painkiller fentanyl has donated $500,000 toward defeating a ballot initiative that would make recreational use of marijuana legal under Arizona law.

It's hard to imagine a more sinister donor than Insys Therapeutics Inc. in the eyes of pot legalization proponents, who long have claimed drug companies want to keep cannabis illegal to corner the market for drugs, some addictive and dangerous, that relieve pain and other symptoms.

Insys currently markets just one product, according to an August filing with the Securities and Exchange Commission: a sublingual fentanyl spray it calls Subsys.

Two former company employees pleaded not guilty last month to federal charges related to an alleged kickback scheme to get doctors to prescribe Subsys.

Illinois Attorney General Lisa Madigan filed a lawsuit late last month against the company alleging Insys hawked the drug to doctors for off-label prescribing, saying the company's "desire for increased profits led it to disregard patients' health and push addictive opioids for non-FDA approved purposes."

Fentanyl is a synthetic opioid significantly more potent than heroin that can cause overdoses, especially when it's used to cut the supply of illegally sourced drugs. The musician Prince died from an accidental fentanyl overdose in April.

Insys made its large contribution to the anti-legalization campaign group Arizonans for Responsible Drug Policy on Aug. 31, according to information posted online by the Arizona secretary of state's office.

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Arizonans for Responsible Drug Policy campaign manager Adam Deguire tells U.S. News that legalization foes will not return the donation. In a statement he expressed gratitude and stressed that Insys is based in Arizona, unlike the Marijuana Policy Project, which has contributed substantially toward passing the initiative.

A voicemail requesting comment from Insys was not immediately returned.

Advocates for the marijuana legalization initiative Proposition 205, which is up 10 percentage points in a poll released Wednesday by the Arizona Republic, condemned the donation.

J.P. Holyoak, chairman of the initiative-backing Campaign to Regulate Marijuana Like Alcohol, said in a statement that "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."

Holyoak said: "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."

From 2011 through at least last year, Insys also sold a second product: a generic equivalent to Marinol , a synthetic version of the cannabinoid THC (tetrahydrocannabinol), which the Food and Drug Administration allows for treatment of cancer and HIV-related symptoms like nausea and loss of appetite, which cannabis advocates say the raw plant material can treat without a corporate middleman. Insys said in its August filing it has no plans to resume those sales, though it is preparing a similar drug.

Marijuana legalization supporters say legal access to cannabis, which does not cause overdose deaths, could help combat overuse of medicine that serves as an on-ramp to potentially life-destroying addictions.

Johns Hopkins University researchers concluded in 2014, after studying the effects of state medical marijuana laws through 2010, that "medical cannabis laws are associated with significantly lower state-level opioid overdose mortality rates."

Currently, 25 states have laws allowing residents to use marijuana as medicine, though the plant remains a federally illegal Schedule I substance, which is defined as having no accepted medical value.

The Obama administration has allowed states broad leeway to regulate sales of marijuana for recreational or medical use, despite federal prohibition, and major-party presidential candidates Hillary Clinton and Donald Trump have indicated they would pursue a similar approach.

Arizona voters legalized medical marijuana in an initiative that won by about a 4,000-vote margin in 2010, and supporters of this year's legalization initiative expect the ultimate vote to be close again, perhaps the closest of the five states – including Maine, Massachusetts, Nevada and California – that also will consider joining the four pioneering pot legalization states and the nation's capital in November.