Inside a hip Lower East Side cocktail bar, late on a recent Saturday night a crowd of young party-goers formed a line outside the bathroom as punk music blared in the background. That’s when one local restaurant worker cut through the noise with a warning: “If anyone is using drugs tonight, do it safely and test!” This has become the unsettling new reality both in and out of the party scene in New York City as fentanyl, the synthetic opioid responsible for about 150 overdose deaths in the US a day, has infiltrated nearly every aspect of the illicit drug...