Fentanyl's rise in the US drug market is part of a much bigger, longer-term strategy from global trafficking groups, according to a new report.
The drug - 30 to 100 times stronger than heroin - is driving the rise in overdose deaths in the US.
It is sold straight, but it is also cut into all kinds of drugs, from heroin to Percocet, and there is no way of users detecting whether it's there until they try it.
Five years after the US declared an addiction epidemic, things are turning around, with prescription rates on the downturn and more education about the risk of addiction from prescription drugs.
But the one thing that still stumps health officials is fentanyl: how did it spread so far in the US, and why do Americans take it?
According to a new report, published today in the journal Addictions, there isn't (and never has been) significant user demand for fentanyl specifically among US drug users.
More importantly, it seems, fentanyl was the easiest and cheapest drug for manufacturers and traffickers to produce fast enough to meet America's rapidly rising demand for opioids.