Starting Friday, Ultra Music Festival is expected to attract more than 160,000 young people from across the world to party with hundreds of international DJs and music artists.

This year, it’s attracting something else: Molly.

Molly is a party drug that is a derivative of Ecstasy, which has fueled dance parties for decades. It appeared on the dance-music and hip-hop scenes around early 2011 — billed as pure MDMA, the amphetamine that is the prime ingredient of Ecstasy. It comes as crystals or as a white powder inside a capsule and can cause high blood pressure, a rapid heart rate, possible brain injury and even heart attacks.

“There’s a lot of psychiatric long-term effects, and certainly there’s injury that you can do to your brain,” said Dr. Jeffrey Bernstein, medical director of poison control at Jackson Memorial Hospital. “We don’t even know all the long-term effects because the drug has only recently been re-popularized.”

It’s popular in electronic music clubs in Miami, and police are expecting the drug to be rampant at Ultra, the country’s largest electronic music festival. This year for the first time, Ultra will run for two full weekends: Friday-Sunday and March 22-24 at Bayfront Park in downtown Miami.

The drug, sold for $10 to $25 a capsule, is a new way to market a familiar drug. It got a memorable boost last year at Ultra, when Madonna went on stage and shouted to the audience: “How many people in this crowd have seen Molly?”

“Molly is a new phenomenon. Something that is still developing,” said Maj. Jorge Martin, commander of the Miami Police Special Investigation Section, which deals with high-end illegal narcotics.“When Madonna made her ... remark, we were starting to see it. A year later, our investigation has grown twofold.”

If Molly started out as pure MDMA, it often isn’t now. Miami Dade police have found that Molly also can contain methylone, a chemical found in bath salts. But whatever its makeup, Molly can cause trouble.

A 2012 study done by three researchers at the University of Miami Miller School of Medicine found three young men treated at Jackson’s emergency room suffered brain hemorrhages after ingesting Molly. None of the three cases — all men in their 20s — had any previous brain abnormalities. Usually, patients who hemorrhage after using other drugs, such as cocaine or crystal meth, had a pre-existing neurological condition.

“We have this anecdote of three patients who have no other reasons to have hemorrhages in the brain after a short period after they had Molly. They had hemorrhages just with the ingestion of the drug,” said Ronald J. Benveniste, assistant professor of Clinical Neurological Surgery at the UM Miller School of Medicine and one of the authors of the study. “We wanted to make the point that Molly is not safe — even if it’s allegedly purified or an actual purified form of MDMA.”

Miami police reports show Molly was confiscated and submitted for testing 207 times in 2011. Of those tests, 190 substances contained MDMA, while only 17 contained methylone. By 2012, the police confiscations were up to 337, a 63 percent increase. Testing showed 278 samples contained methylone and 59 contained MDMA.

MDMA and methylone have similar effects on the body, although methylone and other bath salts can lead to depression and suicidal thoughts after the drug is out of the body, especially if the user has an underlying mental illness.