Dark heroin cut with so much white powdered fentanyl that it’s known on the street as “gray.” Cocaine laced so frequently with fentanyl that club DJs stock anti-overdose medication. Fake prescription pain pills that are in fact all fentanyl.

The synthetic opioid fentanyl, a legal prescription pain medication, is now a black market commodity blasting through the street drug marketplace. Cheap and up to 100 times more powerful than naturally derived opioids, it is also lethal.

Behind the trend is a growing body count: In the 12-month period that ended in April, more than 100,000 Americans, a record number, died from overdoses, according to preliminary data from the Centers for Disease Control and Prevention. The majority of the deaths were linked to synthetic opioids.

While the mounting deaths show the devastating consequence of fentanyl’s seep, it is less widely understood why the drug has mushroomed. And why so many illicit products now contain it.

The spread of fentanyl has been stealthy, steady and deadly, according to interviews with nine people involved in the sale of illegal drugs in New York, where much of the country’s fentanyl enters the street market, as well as law enforcement and addiction experts. The identities and backgrounds of the nine people were confirmed by The New York Times through their criminal records, lawyers and addiction counselors.

People who intermittently use stimulants like cocaine, for example, have low tolerances for such powerful synthetic opioids, said Dr. Chinazo Cunningham, executive deputy commissioner of the New York City Department of Health and Mental Hygiene.

In 2015, just 17 of the city’s overdose deaths involved cocaine and fentanyl, without heroin; that number rose to 183 in 2019, the last year for which data was available, according to the Health Department.

“These are no longer street drugs,” said John Tavolacci, executive vice president and chief operating officer of Odyssey House, a drug rehabilitation center in New York City. “This is poison.”

Fentanyl is the third wave of an opioid epidemic that began in the 1990s with prescription pills, followed by exploding heroin use.

Now communities are struggling under an onslaught of fentanyl. The reasons are multilayered: As pharmaceutical companies have tightened the tap on prescription pain pills following a raft of legal losses for their role in causing the opioid epidemic, the pills have become scarce on the black market. Addicts have turned to fentanyl for their fix.

To profit off the situation, cartels and small-time manufacturers have flooded in caches of imitation pills — fentanyl tablets mimicking prescription brands. In September, the Drug Enforcement Administration issued a public safety alert: More than 40% of black-market prescription pills contain lethal amounts of fentanyl.

“Like a wolf in sheep’s clothing, these pills are lethal,” Ray Donovan, the DEA’s special agent in charge of the New York division, said in a statement.

As borders were closed to thwart the coronavirus pandemic, cartels created stockpiles, leading to a spike, said Bridget Brennan, New York City’s special narcotics prosecutor.

At the same time, several drug dealers said in interviews, domestic dealers turned to fentanyl as a cheap way to bulk out thin wares.

As lockdowns lifted and border crossings began to normalize, fentanyl flooded in. In just the first six months of 2021, the special prosecutor’s office confiscated more than in any previous year.

Since 2018, fentanyl seizures by the New York DEA have tripled, as confiscated heroin fell by more than half. The drug agency in New York says it has taken more than 2,400 pounds, of fentanyl off the street so far in 2021, compared with almost 957 pounds of heroin.

Fentanyl was first synthesized in 1959 as a substitute for morphine. The synthetic opioid is prescribed to treat pain, including in cancer patients. It is often administered in a patch; abusers figured out how to chew or smoke the patches or adhere strips of them to their gums.

Today the drug is far simpler to obtain. Fentanyl is primarily manufactured in China, which sends it or the raw ingredients, called precursors, on cargo ships to Mexico, where it is finished by cartels, according to Ben Westhoff, the author of “Fentanyl, Inc. How Rogue Chemists Created the Deadliest Wave of the Opioid Epidemic.” It is widely available on the “dark web,” an untraceable online network, and shipped in the mail.

Fentanyl’s spread has been pushed by the profit imperative, according to interviews with dealers: On each leg of the journey of a drug like heroin or cocaine, from cartel to end user, sellers often cut the pure product with cheap powders that are similar in appearance, a process known as “stepping on” the drug. Once it was things like baby formula; today, it is likely to be fentanyl.

There is no quality control: A street dealer might cut fentanyl into cocaine that already contains it, creating a lethal dose.

In interviews, dealers described lacing as completely ad hoc.

One said she measured out fentanyl with a McDonald’s ice cream spoon, leveled with a playing card. More than one dealer did not measure at all, spritzing liquid fentanyl onto baking sheets of marijuana, creating a once-rare concoction that some dealers say is increasingly requested.

Anna, a 31-year-old from Brooklyn, was first introduced to fentanyl three years ago while working in a drug mill in Crown Heights, scooping the chemical mixture into glassine envelopes of heroin.

Soon, straight fentanyl became her drug of choice. It was powerful — just three baggies replaced her nine daily of heroin — but above all, lab-made fentanyl costs a fraction of the price of natural opiates, which are derived from poppies.

“I knew it was so dangerous,” said Anna, who said she overdosed twice and was revived with Narcan, an anti-overdose medication. “But I didn’t care. It was so cheap.”

She is now in inpatient addiction recovery.

When Swainson Brown, 40, a beloved chef at a restaurant on the eastern end of Long Island, consumed cocaine in August, authorities said he had no idea it was laced with fentanyl.

Brown was among six people in the community killed in a three-day span by the drug.

“He did not want to die,” said Glenn Petry, Brown’s friend and employer. “That was without question the farthest thing that he imagined would happen to him that night.”