A drug company which produces a painkiller 50 times more potent than heroin has donated $500,000 to the campaign to keep marijuana illegal in Arizona.

Insys Therapeutics - whose sole product is the powerful painkiller fentanyl - gave the money to 'Arizonans for Responsible Drug Policy'.

Their donation was nearly four times more than the second largest of $110,000.

Pro-legalization campaigners have claimed drug companies want to keep cannabis illegal to corner the market for drugs that relieve pain and other symptoms.

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Insys Therapeutics - whose sole product is the powerful painkiller fentanyl - gave the money to 'Arizonans for Responsible Drug Policy'

The company said they opposed the initiative 'because it fails to protect the safety of Arizona's citizens, and particularly its children.'

It also said the US Food and Drug Administration had not approved marijuana for any medical use, the Arizona Daily Star reported.

But activists have seen their move as being a sinister ploy to maintain control over the drug market.

The company produces the painkiller that the singer Prince died from after taking an overdose.

America's Drug Enforcement Agency (DEA) said fentanyl - which the company sells as a spray - is 50 times more potent than heroin.

On its website it says the drug 'may provide pain relief in as little as five minutes...it is the first and only sublingual spray for breakthrough cancer pain.'

However the DEA said the drug was also being used on the streets to cut heroin.

Prince's cause of death was an accidental painkiller overdose of the painkiller fentanyl

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

He added: '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 Proposition 205, the voters should know that it was paid for by highly suspect Big Pharma actors.'

Fellow campaigner Barrett Marson said their opponents should return the donation.