Fentanyl, which looks like heroin, is a powerful synthetic painkiller that has been laced into heroin but is increasingly being sold by itself — often without the user’s knowledge. It is up to 50 times more powerful than heroin and up to 100 times more potent then morphine. A tiny bit can be fatal.

In some areas in New England, fentanyl is now killing more people than heroin. In New Hampshire, fentanyl alone killed 158 people last year; heroin killed 32. (Fentanyl was a factor in an additional 120 deaths; heroin contributed to an additional 56.)

“It sort of snuck up on us,” said Detective Capt. Robert P. Pistone of the Haverhill Police Department in Massachusetts. He said that a jump in deaths in 2014 appeared to be caused by heroin, but that lab tests showed the culprit was fentanyl.

Fentanyl represents the latest wave of a rolling drug epidemic that has been fueled by prescription painkillers, as addicts continue to seek higher highs and cheaper fixes.