Drug company which makes the powerful painkiller fentanyl gives $500,000 to the campaign to keep marijuana illegal in Arizona

• Insys Therapeutics produces the powerful painkiller fentanyl, which is 50 times more potent that heroin
• Company donated $500k to campaign to keep cannabis illegal in Arizona
• Activists said it was a sinister ploy to maintain control over drug market
• Singer Prince died after taking an overdose of the painkiller fentanyl