Fentanyl is often used as a form of cheap synthetic heroin, or cut with heroin.

Apparently a fentanyl high is similar to a heroin high, but fentanyl carries a much greater risk of an overdose.

My best friend in Detroit (I don't live there anymore) died from a hit of fentanyl that was sold to him as heroin. He was selling a story to Vice Magazine about the amount of dangerous fentanyl on Detroit streets being sold as heroin when he died. It was so fucking ironic.