NEW YORK — After a long and raucous dinner party on a recent weeknight, guests decamped to a loft in Manhattan’s TriBeCa neighborhood, perhaps the most desirable chunk of real estate in the city that never sleeps.

There was no music or dancing at this after-party, though. Instead, a host distributed clear capsules of tiny white crystals that guests proceeded to swallow — an illicit dessert known as Molly, a synthetic stimulant that has suddenly become as much a part of the 24-hour-a-day New York lifestyle as cocaine was to another generation.

In this case, two financiers at the party had just completed a multimillion-dollar pact, a cause not only for celebration but for bonding of the kind that only can occur in the netherworld after 3 a.m. Later that morning, everyone drifted out to go to work.

“In today’s era, everyone is popping pills,” said a fashion company owner who was one of the guests but who feared using her name could put her job at risk. “Everyone wants to come to New York and succeed, but there’s so much pressure, so much competition. With Molly, you’re happy, you’re free, there’s no worries, no negative talk.”

ER visits and deaths

Molly is marketed as a pure form of MDMA, the main ingredient in the street drug Ecstasy. Often associated with electronic dance festivals and shilled as a “safe” high, it is gaining fans across the country. But the national Drug Abuse Warning Network noted a 120 percent increase in the number of emergency room visits involving MDMA from 2004 to 2011.

Public health officials say the drug is often not as pure as its marketing claims, and its effects are increasingly proving fatal. Two people died at the New York Electric Zoo festival over the Labor Day weekend after ingesting Molly, and at least four others were hospitalized in critical condition. Two other deaths that weekend, one in Washington state and the other in Boston, were also attributed to the drug.

Molly was a fixture at this year’s Coachella Valley Music and Arts Festival, and is increasingly part of the Los Angeles club scene. Sam Torbati, co-chairman of the Department of Emergency Medicine at Cedars-Sinai Hospital, said Southern California physicians had seen an uptick in patients who had used Molly or MDMA, though precise numbers had not been compiled. In San Francisco, MDMA-related emergency room visits were up 30 percent between 2008 and 2011.

“We are definitely seeing an increase in the number of people who say they’ve taken Molly,” said Dr. Theodore Bania, director of research and toxicology at the emergency departments of St. Luke’s and Roosevelt hospitals in New York.

Jean Mone, a therapist and substance abuse counselor in New York, says that in the last year or two, more of her clients have mentioned using the drug. They range in age, but some of them are in their early 20s and take the drug to stay up all night and blow off steam after long, demanding days at work. Some take Molly to stay up at night and then pop Adderall, another stimulant, to get up in the morning and go to work.