It's late Wednesday in the hipster redoubt of factory lofts and retro bars known as Greenpoint, Brooklyn. Paul, a 35-year-old publishing executive who plays in an indie band on the side, is getting the eye wobbles. He has just come from the recording studio, and he really should be getting home to his wife. But he's trying to make his night last. He retreats to the restroom with a little baggie bearing the warning not for human consumption and snorts a white powder up his nose. He comes back animated and happy—not just happy, but bursting with joy and affection. He proceeds to gush about his love for his bandmates and for his shitty absentee father. His three friends, their own noses packed with powder, sit in rapt attention, nodding rapidly, as alert and empathetic as fans at a Sex and the City 2 premiere.

This is your brain on plant food.

In less than three years, a new drug sold over the Internet as a boutique alternative to Miracle-Gro but purchased solely as a designer high has swept from Israel through Australia and into the club scene in London. It has only recently landed in the United States, but it has already acquired the obligatory street names: sunshine in the Pacific Northwest; star dust in the Midwest; drone and bubble here in New York. It is mephedrone, or 4-methylmethcathinone, MCAT for short—an acronym that prompted the British press to call it meow meow. Chemically, mephedrone is similar in effect to khat, a shrub whose leaves are chewed by Sudanese warriors to achieve an amphetamine-like high before they head into battle. But as drugs go, mephedrone is a lover, not a fighter—the powder energizes, though not as fiercely as cocaine, and that adrenaline jolt is topped with the warm fuzzies of an Ecstasy trip.

"I'm not even a drug person," Paul says after noting the dust's effects on him. (Like the other users in this story, he did not want his last name used.) "But this stuff is so smooth and easy. It doesn't leave your soul cored out."

Until this spring, mephedrone was legal—and thus rampant—in the U.K. Oxford undergrads snorted it off bars. Club kids rolled on it until the sun rose over Clerkenwell. Then users began to get sick. "We started seeing it pop up 17 months ago," says Dr. Paul Dargan, a clinical toxicologist in London and a member of the British Pharmacological Society, whose unit has identified more than 60 cases of mephedrone poisoning. "The pattern of toxicity is similar to Ecstasy and cocaine agitation—fast heart rate, high blood pressure, chest pains, occasionally seizures. Nobody knows how much they take. You don't know how much is in a line."

After several high-profile deaths were attributed (rather prematurely and inconclusively) to mephedrone by the British tabloids, U.K. authorities rushed to ban the drug. But the truth is, no one really knows what risks, short- or long-term, mephedrone presents. Hardly anyone has studied it. "There have been many media claims," Dargan says, "but very few clinical facts."

In the U.S., the Drug Enforcement Agency has scarcely heard of mephedrone. And most of the country's police departments and health-care workers know nothing at all about it. But chances are they soon will. In the U.K., the press has had a field day with the fiendish stories surrounding its use: the weeklong chest pains; the man whose knees turned blue for days; the guy who tore off his scrotum with a pair of pliers after hallucinating that centipedes were crawling all over his body. Thanks to the Reefer Madness-like fearmongering, the myth of mephedrone has mushroomed.

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Most designer drugs come and go, lost in the literal buzz of more-established vices like coke, Ecstasy, meth, and even LSD. But mephedrone caught a lucky marketing break. The drug is believed to have been invented in an Israeli lab in 2007, and it quickly became the dust that fueled the Tel Aviv party scene. Unlike the U.S., Israel has no analog-drug law, which allows authorities to criminalize a new substance if it has effects similar to those of a known narcotic. In Israel you can tweak a singular irrelevant chemical component of meth and declare the result something entirely unique and legal. "Israel, to my sad dismay," says Mickey Arieli, the director of the pharmaceutical-crime unit in the country's Ministry of Health, "is a major leader in designer drugs for this reason."

Though Israel shut down Neorganics, the company that produced mephedrone, in 2009, the drug had already begun popping up on the Internet, sold by copycat manufacturers in China and India. In June 2008, Australian and Cambodian drug agents had seized and burned 1,278 barrels of sassafras oil, the base ingredient of MDMA, or Ecstasy, in Cambodia's Cardamom mountains, causing a shortage of the drug in Europe and Australia, where its use is rampant. Mephedrone became a trendy replacement.