On Thursday, the New York City Health Department announced that it had seen a sharp increase in fatal overdoses involving a combination of fentanyl and cocaine over the last year, and had warned thousands of city health providers to educate any patients who use cocaine casually.

Fentanyl is an opioid similar to morphine, but it's completely synthetic and about 50 to 100 times more potent, according to the National Institute on Drug Abuse. It's also cheaper and stronger than heroin, making it more desirable to some users and dealers, and significantly more deadly. It's often connected to heroin overdoses, and according to the Wall Street Journal, some users even seek out fentanyl to "spike" their drugs.

But now New York City health officials are worried that more recreational cocaine users are falling victim to fentanyl because it's being laced with the drugs they're buying—often without them knowing. According to the press release, more than 1,300 people died of a drug overdose in New York last year—and of those, 37 percent had both cocaine and fentanyl in their system, up from just 11 percent in 2015.

"All New Yorkers who use drugs, even if only occasionally, should know their drugs may be mixed with fentanyl," Health Commissioner Dr. Mary T. Bassett said in the press release.

Cities in Ohio, Canada, and California have also reported cases of people overdosing on fentanyl-laced coke. In New York, the Health Department offers a course in how to administer naloxone, which reverses the effects of a fentanyl overdose. New Yorkers can also pick up naloxone at most pharmacies without a prescription.

But while it's possible to treat those who OD on fentanyl, it's a difficult drug to wipe out. In 2016, 44 percent of all overdose deaths in New York involved fentanyl—compared with just 16 percent the year before, according to the press release. On top of that, more people in New York died from overdoses than homicides, suicides, or car crashes combined.