Pharmaceutical fentanyl is a synthetic opioid, approved for treating severe pain, typically advanced cancer pain. It is 50 to 100 times more potent than morphine. However, illegally made fentanyl is sold through illicit drug markets for its heroin-like effect, and it is often mixed with heroin or other drugs, such as cocaine, or pressed in to counterfeit prescription pills.

These illegal pills look like real medication but often contain powerful doses of fentanyl. Pills from unknown sources may be toxic or even deadly.