Photo

Anthony Rainey, standing, near the Metro-North Railroad station at 125th Street and Park Avenue. Mr. Rainey, who sleeps in a plaza under the station, does not mind the addicts — he smokes K2 himself — but he says the crowds are making it difficult to sleep at night.Credit Josh Haner/The New York Times

It was early afternoon when the man with the twitching legs was dragged from the ground into an ambulance. Another man selling books washed away the vomit.

A man named Charlie Medina sat at the same spot a few days later, unable to remember his name before he fell into a trance with his jaw open and his eyes dilated.

And the lovers. They were unable to find a room. One pulled off the other’s shirt and her bra, then started to kiss her bruised breasts while a small crowd gathered to watch.

The people here on this stretch of 125th Street in East Harlem may change, but the drug remains the same: K2, also called synthetic marijuana, a potent mix of herbs and chemicals that has become widely used among homeless people in New York City.

A joint of K2 goes for a dollar or two, far cheaper than food. Many bodegas on 125th Street sell it. A marijuana joint, by comparison, costs about $5. Crowds of up to 80 or 100 homeless people come in on buses from a nearby shelter on Randalls Island, drawn by heroin recovery clinics nearby, and spend the day here under the influence of this cheaper narcotic. The block between Park and Lexington Avenues appears at times to be a street of zombies.

“This is a K2 nation out here,” one man says before walking away confused.

Nearby blocks tell a different story. A Whole Foods grocery is under construction on 125th Street. An upscale restaurant run by the celebrity chef Marcus Samuelsson serves expensive soul food dishes nearby. But this stretch is a holdout of what Harlem looked like before the development push began — perhaps an even darker version of that time. It is also a visual tableau of the homeless problem that has dogged the administration of Mayor Bill de Blasio.

The drug comes as a plastic bag of herbs sprayed with chemicals known as cannabinoids, which work like a deeply potent version of marijuana. K2 is illegal in New York State along with variants with names like Spice, AK-47 and Scooby Snax. But few arrests are made because manufacturers frequently change the chemical mix as substances are banned.

The City Council speaker, Melissa Mark-Viverito, said last week she would introduce a law banning substances marketed as synthetic marijuana or any imitation drug that had a similar effect, a move aimed at stopping manufacturers from skirting the law. Offenders could face up to a year in jail and a fine of $5,000.

At a news conference, she cited a “surge of the use of it in homeless shelters” and cites 125th Street as the epicenter of the problem.

On a recent day, a 47-year-old man who goes by Green stood on 125th Street pushing a plastic straw into his ear. He suffers from Parkinson’s disease and says he has been self-medicating with K2.

“Sometimes it calms the twitching down,” he said, straightening out the straps of a tattered backpack. “Sometimes it just makes me worse.”

Michael Morgan, 46, spends most days under a scaffolding next to an Apple Bank branch on 125th Street, cradling a broken leg in a cast, smoking K2.

The sidewalk has been his home since April, after he says staff members of a nearby shelter forced him to leave when they suspected he was physically abusing his 29-year-old girlfriend, Saida. “It was just a lot of yelling and screaming and throwing objects,” he said. “I was the one who got kicked out.”

The year has been marked by a gradual disintegration of Mr. Morgan’s life. He said the authorities took custody of three children he had with another woman and split the siblings up into different foster homes in the Bronx. He sees them rarely.

Before the trouble with his current girlfriend at the shelter, the two of them would save money from panhandling and go on long drug benders around the city, followed by slow recoveries in shelters and on the streets.

A smile suddenly flashed across Mr. Morgan’s face, exposing the gaps where his front teeth had been. His girlfriend had arrived. She pulled out an envelope filled to the brim with the dry green herbs of K2. She measured out half of it for him, spilling a bit on the sidewalk.

Mr. Morgan rolled a joint and lit up.

“If I weren’t on this I might be angry, I might be hitting her. Who knows what I would be up to,” he said, his words trailing off with each drag of the cigarette. “This stuff makes me calm.”

After a while he came to and looked up at his girlfriend.

“Do you want to stay with me here tonight? We can have some fun in the grass, then sleep under the bench.”

Police raids on bodegas on this street in July ended in confiscations of more than 8,000 packets of K2, but many of the stores continue to sell the drug, those on the street say. That has left police officers looking on as drug users light up in plain sight. Then the officers haul them into ambulances as they drop onto the street.

The sheer number of users has left officers on edge on this block. “It quickly can become a kind of group mentality where the officers, or even multiple officers, are outnumbered,” Deputy Inspector Tom Harnisch, commander of the 25th Precinct, said.

K2 may be the most popular drug here, but it is only part of the black market. Two drug dealers hawk so-called nickel bags of marijuana for $5 below the Metro-North Railroad station and residents point to where heroin is for sale at the top of the Lexington Avenue subway line. A block away, a man offers to buy food stamps for half their value in cash.

“Is anyone here selling a baby?” a couple asked a group of homeless people on a recent afternoon.

“A baby?” a man yelled back, confused.

“We heard a baby was for sale,” the man replied. For a moment there was silence, then the couple laughed.

“Don’t mind us; we’re on acid,” the girlfriend said, and the two staggered toward the subway station, grinning and drawing circles in the air.

For those on this block who are mentally ill, the drugs seem to close off most hope of recovery.

Demond Outten, 40, said he began using K2 after being released in April from the psychiatric ward of St. Barnabas Hospital in the Bronx.

Tall and dressed in tattered clothes, Mr. Outten still wore the armbands from his four-day lockdown for schizoaffective disorder and ADHD. He said the drug helps him to practice what he calls his “Rosicrucian alchemy.”

He pointed to the sum of his belongings, gathered up in a shopping cart: a box of detergent, a stack of playing cards, a toothbrush and a takeout box from several nights before. He muttered a few words about his son Albert, a 2-year-old who was nowhere near; and about a girlfriend who he said punched him in the eye. He became angry and walked away.

Late that night as others slept, a man wearing only suspenders and jeans flapped his arms in the air as if he were trying to fly. People call this man Man-Man, and he would not give a reporter any other name.

The scene is not as bleak everywhere on 125th Street. There was Oscar Velez, 35, a former K2 addict who lost his job as a construction worker recently. On a recent day, he walked the street asking those he knew there if they were still using K2.

“I scold people now,” he said after an afternoon visit to a nearby methadone treatment center for his heroin addiction.

To demonstrate, he cornered Camacho, a friend he says he met in a Puerto Rican jail gang whom he had seen “standing frozen on the stairs of the subway denying he was on drugs.” The friend shrugged off the interrogation, smiling, and walked away. Mr. Velez says he is slowly making a difference.

But he admits the temptation of the drug is real when he comes back to 125th Street, especially since he has no work. A nearby bodega offered him money to distribute K2. He thought about it, but turned the offer down.

Anthony Rainey, who sleeps in a plaza under the Metro-North station, remembers 125th Street before K2, having lived here since 1971. The most popular drug before was marijuana, he says, and before that, cocaine and heroin. But those drugs were expensive, he says, and “this only costs dollars.”

“It will last you all day,” Mr. Rainey said. “Now the ambulance comes by and picks up four or five people a day.”

Mr. Rainey does not mind the addicts — he smokes K2 himself now — but he says the crowds are making it difficult to sleep at night. And the nearby construction bothers him: a mixed-use development next to the Metro-North station scheduled for completion in 2017, according to a sign.

“When that finishes,” Mr. Rainey said, “we’re out of here.”

On a recent afternoon, Mr. Morgan, who also sleeps on 125th Street, talked about plans to make some money, a scheme that involved distribution of K2. He had recently discovered he could buy a pack of the drug for $5, roll seven joints, and sell each for a dollar to the other homeless people there. The profit was $2 per bag.

“And if you invest some money, $50, imagine how much more you could get,” he said. “Imagine what you could do with that.”

Mr. Morgan started to trail off again. He had taken the drug and it was starting to kick in.