Of the five neighborhoods with the highest opioid-related overdose rates in 2015 and 2016, four were in the Bronx — Hunts Point-Mott Haven, Crotona-Tremont, High Bridge-Morrisania and Fordham-Bronx Park — and one was in Staten Island, South Beach/Tottenville.

The crisis in the Bronx stems, at least in part, from a surge of opioids in a place where some residents have long struggled with addiction. Heroin has become much cheaper in recent years as the supply in the United States has grown, according to the Office of the Special Narcotics Prosecutor for the City of New York, and individuals with histories of drug abuse are particularly vulnerable to relapse amid a surge of cheap drugs. It has also become significantly more potent.

The cheaper, stronger heroin has been made even more dangerous by the proliferation of fentanyl, which is 50 times more powerful than heroin. Interviews with nearly 200 drug users conducted by the city health department suggest that most users are not directly seeking fentanyl; narcotics experts say the drug is likely being mixed into heroin batches, often without the dealers themselves knowing, let alone users. As effective as naloxone can be in reversing overdoses and restoring breathing, fentanyl overdoses are often too extreme for the antidote to work. And naloxone is ultimately a Band-Aid to a broader, systemic addiction crisis across the city.

“In a place like the Bronx, where there was a long term underlying addiction issue, all of a sudden you saturate the area with cheap accessible heroin, and you’re going to start to see the spike,” the city’s special narcotics prosecutor, Bridget Brennan, said in an interview.