When powerful street drugs collectively known as synthetic pot are smoked, the resulting high mimics the effects of marijuana. Yet these man-made cannabinoids are not marijuana at all. The drugs, more commonly called spice, fake weed or K2, are made up of any number of dried, shredded plants sprayed with chemicals that live in a murky legality zone. They are highly dangerous—and their use is on the rise.

Synthetic pot, which first hit the market in the early 2000s, has especially caught the attention of public health officials in the past couple of years, stemming from a surge in hospitalizations and violent episodes. Although the drugs act on the same brain pathway as weed's active ingredient, they can trigger harsher reactions, including heart attacks, strokes, kidney damage and delusions. Between June and early August usage of these drugs led to roughly 2,300 emergency room visits in New York State alone. Nationwide more than 6,000 incidents involving spice have been reported to U.S. poison-control centers this year—about double the number of calls in 2013.

Ever changing recipes make it possible for spice sellers to elude the authorities. Each time an ingredient is banned, producers swap in another compound. The drugs are then sold on the Internet or at gas stations and convenience stores at prices lower than genuine marijuana. The changing formulations also pose a challenge for researchers trying to match the chemicals with their side effects or to develop tests to identify them in a user's system. “The drugs are present in blood for only a short period, so it's very difficult to detect them,” says Marilyn Huestis, chief of the Chemistry and Drug Metabolism Section at the National Institute on Drug Abuse. Huestis is now working to identify synthetic cannabinoid by-products via a method that captures all ions present in a single test sample. It can take a month to evaluate one compound, but to keep up with the influx of pot knockoffs, she says, “I think this [method] is our only hope.”