Ignoring the police officers standing down the block and the disingenuous fine print on the foil packet peeking out of his front pocket — "Warning: Don't Smoke" — a homeless man openly lit up a synthetic marijuana joint and explained why it's not like the real thing.

"It's a zero-to-60 high," said the 47-year-old, who gave his name only as J.C. because of his frequent run-ins with the law. "I've done plenty of drugs in my life, and it only compares to dust," he said, referring to PCP. "But it doesn't last as long."

The tutorial was offered in broad daylight on a bustling street corner in East Harlem, one the neighborhoods where the New York Police Department says it's seen an alarming increase in consumption — mainly by homeless men — of the leafy substance known as "K2." The cheap knock-off weed is spiked with unknown chemicals that are supposed to mimic the more mellow effects of pot, but often comes with harsh side effects that have created a quandary for authorities already grappling with how to deal with the city's homeless population.

"When people talk about synthetic marijuana, it's kind of bad misnomer because we don't know what these chemicals are," said Robert Messner, a police official in charge of civil enforcement.

What's known is that in recent months, there's been a spike in emergency room visits in New York City by users suffering from high blood pressure, hallucinations, hot flashes and psychotic meltdowns that can turn violent or deadly.

On July 24, five patients at a psychiatric facility on Wards Island off of Manhattan were rushed to the hospital after smoking synthetic marijuana. Less than a week later, a man in the West Village jumped into the Hudson River and drowned. A friend told police the victim was high on K2.

New York City health officials issued warnings in April after synthetic marijuana sent 160 people to hospitals in a little over a week. Statewide, there have been more than 1,900 emergency department visits from April through June alone, prompting Gov. Andrew Cuomo to call for tighter regulations on an existing list of banned substances to include new chemical compounds.

The risks of synthetic marijuana aren't limited to the smokers: An internal NYPD memo issued last month warned officers that some people strip off their clothes, become impervious to pain and go berserk if confronted, and advised to call for backup and use a Taser if necessary to get them off the street. At a recent new conference, Police Commissioner William Bratton described how a suspected user who locked himself inside a home and began tearing it apart suffered a gruesome injury when he deliberately grabbed the blade of an electric saw that emergency service officers were using to get him out.

Users can go "totally crazy," Bratton said. "Some of the normal takedowns we use aren't going to work. ... It's something we're very concerned about."

Worries over synthetic marijuana aren't new or confined to New York. In 2013, Washington DC launched a zombie-themed website — K2ZombieDC.com — to warn teenagers of its dangers. Earlier this year, the National Association of Attorneys General wrote to gasoline companies to demand that they outlaw sale of synthetic drugs — which can come in the form of herbal incense and potpourri — at gas stations and convenience stores.

Authorities in New York have largely treated the trend has a public health issue, with police officers calling ambulances for users in distress, sometimes after handcuffing them for their own safely. But they've also sought to put a dent in the market by using health codes to raid small businesses to issue and seize thousands of packets of K2 believed to be produced in China — under brand names like "Green Giant," ''Smacked" and "AK47" — that go for as little as $5.

In East Harlem, the packet carried by J.C. was called "What's Up?" and — despite the warning not to smoke its contents — had the wording, "lab certified, no banned chemicals," and "it's legal." J.C. and other homeless people gathered on the corner of 125th Street and Lexington Avenue pointed to a deli and a smoke shop where they said K2 was sold, though none was on display in either location that day and workers denied having any.

At one point, J.C. gave a pinch of his stash to another man, who rolled a cigarette and smoked it without making an effort to hide it. Nearby, a homeless woman, Victoria Parks, talked about dabbling in synthetic marijuana but preferring vodka. She called the scare overblown.

K2 "changes your reality," said the 32-year-old Parks. "It heightens your senses."

Passer-by Carol Shoemaker, 58, looked on with disgust. The lifelong Harlem resident called the open use of synthetic marijuana a blight.

"They got rid of all the crackheads and here come the K2 smokers," she said. "It's just terrible."