The third day of New York City's Electric Zoo music festival was canceled by the mayor's office this weekend, following the tragic deaths of two twenty-something revelers.

The annual Randall's Island festival typically punctuates the end of summer, drawing more than 100,000 ravers to a dusty, pulsating bacchanal featuring multiple stages and DJs blaring bass-heavy electronic dance music, or EDM. According to police reports, the victims — 20-year-old Olivia Rotondo of Providence, R.I., and 23-year-old Jeffrey Russ of Rochester, N.Y. — allegedly overdosed on "Molly," a potent, supposedly "friendlier" form of MDMA used by millions of partygoers every single year. Here's what you should know about it:

What is Molly, exactly?
Its chemical name is methylenedioxymethamphetamine, but it is more commonly referred to as MDMA — the active ingredient in the party drug Ecstasy. It was first synthesized in 1912 by German chemist Anton Köllisch, but usage of the stimulant didn't really begin taking off until the 1970s, when pharmacologist Alexander Shulgin resynthesized the chemical to share with his friends, particularly psychotherapist Leo Zeff.

Zeff extolled the drug's anxiety-alleviating virtues as a means to get therapy patients to reveal their most buried fears, and was said to have trained 4,000 therapists in how to use it. The drug soon found its way out of psychotherapy and onto dance floors all across Europe, where it immediately became a hit.

Whereas Ecstasy is frequently combined with other, potentially more dangerous drugs — including speed, ketamine, or even LSD — Molly is a fairly recent rebranding effort that is said to contain pure MDMA. Molly is often ingested in a powder of crystal form, and is available illegally for $30 to $50 a dose.

What does Molly do to you?
In The Drug Book, Michael C. Gerald writes that the stimulant "produces euphoria and physical energy, increasing feelings of empathy and intimacy with partners" that makes users feel as if "all is right in the world." The National Institute on Drug Abuse says this is accomplished when MDMA binds to serotonin transporters, which alters the brain's neurochemistry. From a psychological standpoint, this can result in (temporary) heightened perceptions, elevated mood, reduced appetite, and a prolonged burst of energy.

"It makes you really happy," one Molly user tells The New York Times. "It's very loose. You just get very turned on — not even sexually, but you just feel really upbeat and want to dance or whatever."

Where did the name "Molly" come from?
The name is thought to have been derived from the word "molecule." Rusty Payne, an agent at the Drug Enforcement Agency, tells the Times that the term wasn't really used before 2008. Since then, Molly has been "very much glamorized in pop culture," says Payne. References to it have appeared in songs from artists as far-ranging as Kanye West, Rick Ross, Miley Cyrus, and Madonna.

How dangerous is Molly, really?
The adverse long-term effects of MDMA on the brain are still the subject of intense debate among clinical researchers. A notable 2003 retraction in Science said that an alarming study previously published in the journal that suggested a single tablet of ecstasy could cause irreversible brain damage was, in fact, "nonsense." In 2009, Professor David Nutt at Bristol University wrote in an article published in the Journal of Psychopharmacology that the long-term damage of MDMA has been grossly overstated: "There is not much difference between horse riding and ecstasy."

Which isn't to say the drug isn't without its dangers. Gothamist reports that Rotondo reportedly took six hits of Molly this weekend before she fell into a seizure and died.

In 2011, hospitals across the country reported more than 22,000 MDMA-related emergency-room visits, which according to the Drug Abuse Warning Network, is a 120 percent increase from 2004. In The Drug Book, Gerald writes that MDMA can make users overstep the body's natural limits:

After nonstop frenetic dancing for hours in hot, crowded surroundings, some participants experience hyperthermia, a dangerous rise in body temperature that can cause kidney and liver failure. Drinking excessive volumes of water, coupled with dehydration due to sweating, can cause a steep drop in blood sodium levels, potentially resulting in confusion, delirium, and convulsions. [The Drug Book]

Users coming down from MDMA often report feeling depressed, anxious, and fatigued. Mislabeling is also a risk, since Molly is illegal and users oftentimes can't tell the difference.

Young people "will often mix ecstasy with other drugs, especially at parties, like alcohol and marijuana," Dr. Damon Raskin, an addiction specialist, tells CBS News. "I think that the combination of these drugs makes them all the more toxic."