New York City officials report a large influx of the synthetic opioid fentanyl, according to The Wall Street Journal. Fentanyl is 30 to 50 times more powerful than heroin.

In March 2015, the Drug Enforcement Administration (DEA) issued a nationwide alert in response to a surge in overdose deaths from heroin laced with fentanyl. Fentanyl is potentially lethal, even at very low levels, according to the DEA.

Mexican drug-trafficking organizations are increasingly synthesizing their own fentanyl and sending it to New York, officials said. Until recently, fentanyl was primarily purchased off the black market for prescription drugs. As a prescription drug, fentanyl is typically used to treat patients with severe pain, or to manage pain after surgery.

“It is a marked increase,” said James Hunt, special agent in charge of the DEA’s New York office. “A few years back, there were little pockets of fentanyl found in places. Right now, we are seeing Mexican traffickers actually sending fentanyl with loads of heroin.”

Fentanyl seizures in New York state have increased from a half-kilogram in 2013, to 21 kilograms in 2015. So far this year, the DEA has seized 25 kilograms statewide. One kilogram is 2.2 pounds. Investigators are also finding fentanyl mixed with heroin, as well as fentanyl pressed into pill form and sold as counterfeit oxycodone or Xanax.

“A critical part of prevention is just limiting supply,” Bridget Brennan, the city’s special narcotics prosecutor, told the newspaper. “It’s not like building more treatment beds will solve this problem.” She noted that heroin is highly addictive. “You put fentanyl in there and all bets are off,” she said.

Lewis Nelson, a professor of emergency medicine at NYU Langone Medical Center, is seeing more fentanyl-related overdoses and deaths. “It’s devastatingly bad,” he said. “There’s just such a small margin of error, given how potent it is.”