New York City is known for its strange sights. But on 12 July, even locals were shocked by what they saw: more than 30 people staggering around a Brooklyn block with empty stares, shuffling their arms and feet and occasionally groaning. What sounds like the opening of a horror movie was suspected from the start to be the work of a synthetic cannabinoid. Now, a new analysis, out today in The New England Journal of Medicine , confirms those suspicions. But it has also raised scientific ire over its prolific use of the word “zombie.”

Developed by academics and pharma companies to study cannabinoid receptors in the human body, synthetic cannabinoids act on the same receptor on brain cells as cannabis. The compounds, which can be up to 100 times more potent than cannabis, are a rapidly growing class of drugs, usually dissolved in liquid and sprayed on leaves to be smoked. There are hundreds of different compounds, and though they are quickly made illegal in many places, new ones appear every year.

To find out what was responsible for the Brooklyn episode, researchers from the University of California, San Francisco (UCSF), started with a foil-wrapped pouch of herbs found on one of the patients, labeled “AK-47 24 Karat Gold.” When they analyzed a sample, they found it contained the substance AMB-FUBINACA, a powerful synthetic cannabinoid similar to a compound first patented by Pfizer in 2009. The researchers also found breakdown products of AMB-FUBINACA in the blood of eight patients.

But the most interesting thing, says Rainer Spanagel, a pharmacologist at the Central Institute of Mental Health in Mannheim, Germany, is that AMB-FUBINACA seems to have resulted in fewer serious side effects compared with similar compounds. “These highly potent cannabinoids usually have a strong toxic effect on the heart, but that does not seem to be the case here,” he says. The authors also note that its effects on the brain were not accompanied by hyperthermia or acute kidney injury, as is often the case.

But the Brooklyn episode is “nothing special,” says Volker Auwärter, a forensic toxicologist at University Clinic Freiburg in Germany. “We have people coming to the hospital all the time,” similarly affected by synthetic cannabinoids. One of those is MDMB-CHMICA, which has been associated with at least 71 serious adverse events including 29 deaths in eight EU states, according to a 2016 report by the European Monitoring Centre for Drugs and Drug Addiction and the Europol law enforcement agency. Many prisons in the United Kingdom are awash in similar substances, says David Nutt, a pharmacologist at Imperial College London. “We have people go to hospitals with adverse events every day,” he says.

Auwärter and Spanagel also criticized the use of the word “zombie” in the article, which appears once in the title and six times in the text. “It’s frankly surprising that an old, prestigious journal like The New England Journal of Medicine would sensationalize it this way,” Spanagel says. And Auwärter calls it “a bit of a trick to make the story sexier.”