An illegal drug that was originally developed by Pfizer Inc. is behind a spate of overdoses in Brooklyn that sent more than two dozen people to the hospital and turned a city block into “zombieland” this past July.

AB-FUBINACA, which was first patented by the pharmaceutical company PFE, -0.69% in early 2009, was found in a synthetic cannabinoid involved in the July 12 incident, according to a New England Journal of Medicine article published this week.

Pfizer confirmed to MarketWatch that the company never moved forward with AB-FUBINACA. But the public patent appears to have fueled its development in foreign labs, the New York Times reports.

“Years ago we investigated a class of compounds for potential therapeutic value in treating cancer pain and inflammatory pain. Our work in this area was confined to the lab, never tested in patients, and eventually discontinued,” a Pfizer spokesperson told MarketWatch.

The drug is one of a fast-growing class of drugs called synthetic cannabinoids — also called spice, K2 and black mamba — which have become incredibly popular as marijuana alternatives. Also called synthetic marijuana, the drugs were originally developed by U.S. academics and pharmaceutical scientists, then abused as drugs around 2008, according to the NEJM article.

Read: Overdoses on K2 send 33 to Brooklyn hospitals

Users like synthetic marijuana because it’s inexpensive and undetectable on drug tests. But the unregulated drugs, which are classified as “Schedule I” controlled substances, can be incredibly dangerous. Their most severe effects include psychosis, delirium, seizures and death, according to the NEJM article.

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Synthetic marijuana, which is developed in Asian laboratories and distributed globally, is particularly dangerous because the way the powdered drug is mixed with herbal products can, if done wrong, have “possibly devastating effects,” according to the New York Times report, with the Brooklyn incident serving as one such example.

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There’s no known medical use of AB-FUBINACA and other synthetic cannabinoids, according to the Department of Health and Human Services.