The drug is legal and can be prescribed by a doctor for a variety of conditions - most often to treat severe pain.
But it's showing up on the street as well.
Authorities are seizing more and more of the drugs in the US and Canada. The total number of reported fentanyl drug seizures in the US in 2014 jumped to 4,585, from 618 in 2012, according to the New York Times.
And it's just one part of the far bigger issue of prescription painkiller use and abuse: Despite being home to just 5% of the world's population, America consumes 80% of its opioids.
Read on to find out where the deadly drug is being made, what it looks like, and what it does: