Fentanyl is an extremely potent, synthetic (man-made) opioid. It is about 50 times more potent than heroin and 100 times more potent than morphine. In contrast, heroin is 2 to 3 times more powerful than morphine. Fentanyl is a legally prescribed drug for pain in the US and is classified as Schedule II controlled substance when used for legitimate purposes. Heroin is illegal in the U.S. and is classified as a Schedule I controlled substance.