The drug K2 was named by authorities this week as the culprit behind an apparent mass overdose in Brooklyn, New York. More than 30 people were hospitalized Tuesday after law enforcement found them lying on the ground in the neighborhoods of Bed-Stuy and Bushwick, the NY Daily News reported. One witness described it “like a scene out of ‘The Walking Dead.'” Police blamed K2 for Tuesday’s incident. The drug, also called synthetic marijuana or “Spice,” has become an increasing presence in American cities. Here’s what you need to know about it. What it is: K2 is technically made of synthetic cannabinoids, which the National Institute on Drug Abuse defines as “a growing number of man-made mind-altering chemicals that are either sprayed on dried, shredded plant material so they can be smoked (herbal incense) or sold as liquids to be vaporized and inhaled in e-cigarettes and other devices (liquid incense).” It often doesn’t show up on drug tests. What it does: The Drug Enforcement Administration writes that, psychologically, it works like marijuana and can give people paranoia or giddiness. “What K2 does is puts you in a world, a delusional world, have your mind spinning,” a New Yorker identified only as Andrew told CBS News. Where it comes from: We don’t totally know, though the DEA says a lot of sellers have websites registered in China. The institute identifies some of the most popular K2 brand names as Black Mamba, Joker and Kronic. Who uses it: A lot of people. In 2010, more than 11,000 people were hospitalized after using K2. But numbers have skyrocketed recently. Last year, more than 6,000 went to emergency rooms in New York City alone in connection with K2, the New York Times reported. hy it’s dangerous: K2 can cause users to have strokes and exaggerate existing mental illnesses, according to the National Center on Addiction and Substance Abuse. It’s addictive. But the true issue is that because the makeup of each K2 packet varies, the chemicals people are consuming could “contain substances that cause dramatically different effects than the user might expect,” according to the drug abuse institute. How officials are reacting: They’ve been raiding suppliers—last fall, police seized 2 million packets of K2 in the Bronx. However, the drug is difficult to control. “It looks like marijuana, but it’s laced with chemicals and as fast as we are able to identify the chemicals and get that particular chemical, make it against the law, they change the makeup of it,” New York Police Department Commissioner Bill Bratton told reporters last year.