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The little Ziploc bag is clearly labeled. If you miss the message in the words not for human consumption, there's a big red circle with a line through it and some ominous chemical icons. Yet the hipsters at this factory-loft party in the Williamsburg section of Brooklyn aren't bothered by it. Keith, a 35-year-old advertising executive and muralist, digs in with a key and hands the bag to the next partygoer.

"I've been pretty much dipping on this like it was coke," he says sometime around 3 a.m., as the entire room jumps to a live band. "It's not overwhelming—just nice and warm and fuzzy." As a result, he says, he ended up doing a bit too much a few nights earlier and woke with an awful hangover. "I was totally depleted of energy," he says, grinning madly and twitching with the loud music.

According to Camilleri, the Australian forensic scientist, mephedrone's immediate aftereffects vary. People who use large quantities can get vasculitis and vasoconstriction, a tightening of blood vessels, which explains the blue knees and feet in some overindulgers. No one knows how addictive the drug is, but Camilleri says users report the urge to re-dose regularly. Despite the Ecstasy-like touchy-feeliness and heightened sex drive it promotes, 29 percent of users report depression after using it, he adds. And since mephedrone has a pharmacological profile similar to meth's, it's possible that it could cause the same psychosocial problems with long-term use.

Tim, a 32-year-old new-media executive at the same loft party, has his own bag of mephedrone. It arrived via airmail with the usual warnings. Unlike Keith's fine powder, though, Tim's MCAT resembles methlike crystalline chips. He soon gets his own private party under way, taking a single small bump off a kitchen table cluttered with booze bottles. Within seconds, something seems to go terribly wrong. His face turns crimson red and he grips the table, breathing heavily, swallowing hard for five harrowing minutes. This gives way to a wave of euphoria. For the next two hours, he shifts between being wired and withdrawn and being affectionate and blissed-out, rising and falling as if trapped inside the car of a roller coaster. "That was fucking scary," he says. "I thought my heart was gonna explode in my chest."

Two days later, Keith feels drained. "I had the same kind of morning anxiety hangovers I get after a good Ecstasy trip," he says. "But that's always the price you pay. Plus I hooked up with a little hottie at the end of the night who was loving this stuff. So...not too bad."

Nearly a week after the party, Tim is still reeling. He complains of chest pains. He suffers from terrifying, hour-long bouts of anxiety every morning. I ask whether he plans to finish the two five-gram bags he has left since he took only one hit and then was too messed-up to cut lines for anyone else. "Are you kidding?" he says. "I flushed that shit in the East River. Even if I gave it away, it might kill someone. Shit like that—it's Russian roulette."

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