The fentanyl dealer from Los Angeles stands to the side watching carefully as a Mexican drugs cartel operative prepares his latest shipment. The synthetic opioid drug is wrapped in foil, sealed in plastic, then dropped with an oily splash into the petrol tank of the trafficker’s nondescript car.

Jay, not his real name, had crossed earlier from the US to this cartel-run safe house on the Mexican side of the border. The house looks like any other in this neighbourhood. We are told to drive in quickly and an iron gate closes firmly behind us. They don’t cook the drug here, but still they are wary of attracting attention. The men all speak in hushed voices and work quickly.

Their lethal business has become the centre of a dispute causing shockwaves in the global economy after the White House used fentanyl smuggling through US borders as a key justification for raising tariffs. US President Donald Trump has also vowed to “wage war” on the drug cartels.

The BBC gained rare access to a cartel’s operation along the border and travelled to the US to meet their ultimate customers, to see if the international row was doing anything to halt the illegal flow of narcotics.

The men we meet at the safe house are foot-soldiers of a well-known cartel. Two of them loading the car admit to fleeting moments of remorse. But when I ask the man packing the drugs into the fuel tank if he feels guilty about the deaths the pills cause, he sniggers. “We have family too, of course we feel guilty. But if I stop, it’s going to continue. It’s not my problem,” he tells me with a shrug.

The men keep their faces covered while they remove the back seat of the car to gain access to the tank, taking care not to spill petrol. The smell inside the car could alert customs officers on the other side of the border that the fuel tank has been tampered with.

The light green pills, 5,000 in total and marked with an M, are packed tightly – a fraction of what Jay says he sells every week in LA and across the American northwest.

“I try to get 100,000 pills a week, every week,” the softly spoken dealer tells me. “I don’t send them in one vehicle. I try to spread it in different cars. That way I minimise my risk of losing all my pills.”

A 25% tariff on all goods from Mexico was introduced in response to what President Donald Trump said was the unacceptable flow of illegal drugs and illegal immigrants into the US. Some of those tariffs have since been delayed until 2 April.

Defeating the fentanyl trade is one of President Trump’s top policy goals, but Jay doesn’t rate his chances.

“Last time he was in office, he tried to do the same thing, and it never happened. There’s always going to be a demand. And where’s the biggest demand? United States, lucky for us. We’re here in the border,” says Jay with a smile.

There is so much of the drug flowing into the US, most of it coming from Mexico, that according to Jay the price he sells for in LA has fallen from about $5 or $6 per pill a year ago, to $1.50 now (£1.16).

Mexican police say cartels switched in a big way to fentanyl, which is 50 times more potent than heroin, because unlike other opiates – which are made from the opium poppy – it is completely synthetic and much easier to make and transport.

Fentanyl’s strength and addictiveness have left a deep scar on American society: drug overdoses kill more people in the US than guns or car crashes. Fatalities have started to decline, perhaps in part to the greater availability of Naloxone, a drug that reverses the effects of overdoses of opioids. But the latest figures are still stark: 87,000 overdose deaths (mostly from opioids) from October 2023 to September 2024, down from 114,000 the year before.

In an attempt to stave off punitive tariffs from the White House, Mexican President Claudia Sheinbaum has pledged to send 10,000 National Guard troops to the border. The government has made more than 900 arrests since October in Sinola, a major drug trafficking hub. Back in December, Mexico announced its biggest ever fentanyl seizure in the state: more than a tonne of pills. In fact, the country has seized more fentanyl in the past five months than it did in the previous year.

Mexico has also made it harder to import a key ingredient of fentanyl from China, prompting cartels to reduce the strength of each pill – and, in the process, making them less deadly.

And at the end of February, 29 senior drug cartel figures were handed over to the United States, including members of five of the six Mexican crime syndicates that President Trump’s administration recently designated as terrorist organisations.

President Sheinbaum also said she had agreed to the CIA increasing surveillance drone operations over Mexican territory in search of fentanyl drugs labs, after the media revealed the covert missions.

Jay acknowledges the dangers of his trade to himself and his customers, but is untroubled.

“They always try to blame us, that we are the ones that are poisoning American citizens. But they’re the biggest users.

He coolly insulates himself from responsibility and guilt for the deaths his drugs cause. He claims not to know anyone who has died using his product. “I only deal with other suppliers,” he tells me.

The cartels mostly use American citizens to courier their drugs across the border, as they are less likely to be stopped by US Customs and Border Protection. The driver, who goes by the name Charlie, has a US passport. He, too, is mostly indifferent to the suffering the fentanyl epidemic has caused.

“I need the money,” he says. When I ask him how many times he has made the drugs run, he replies: “Too many.” (I later learn that the 5,000 pills in the fuel tank made it across the border without incident.)

President Sheinbaum has also recently emphasised the demand side of the crisis, saying the US fentanyl crisis began with the legal but “irresponsible approval” of painkillers, such as OxyContin, starting in the late 1990s. “The US government should take responsibility for the opioid-consumption crisis that has caused so many deaths,” she said at a daily news conference.

In Philadelphia’s Kensington neighbourhood – dubbed the largest open-air drugs market on the US east coast – Rosalind Pichardo of Operation Save Our City is on to her second Bible. She records in the book’s back pages the number of times she has reversed an opioid overdose using the quick-acting drug Naloxone.

For the past six years, the figure totals 2,931. She flicks through the pages and that number written in red comes alive with the memories of the individuals she saved, and the ones she lost.

She begins to list: “Male in his 60s… male 30s… female in her 30s, very thin, no hair.” Beside each name in this roll-call of fentanyl victims, is the number of doses of Naloxone – sold under the name Narcan – she used to attempt to revive people.

Ms Pichardo, who runs a drop-in centre called Sunshine House, operates what she calls a “no-judgement zone”. She bristles at the terms like “addict”, “junkie” or “zombie”, which have been used to describe the people of her neighbourhood. Instead, she calls everyone “sunshine”.

Some she doesn’t remember; others she will never forget.

“Look at this one, seven years old, two Narcans,” she points out. Ms Pichardo had been called to a neighbour’s house where a woman was holding in her arms a child who had turned blue. Ms Pichardo went inside and the girl was placed on the floor, but as she entered the child’s father ran upstairs carrying a bag. “I’m thinking if that was my child, I’d be running to help the child,” she recalls.

At first, she thought it might be epilepsy, but she spotted drug scales and plastic baggies on a nearby table. The kid’s dad was a drug dealer; the seven-year-old had been poisoned by his stash and overdosed. “I was livid,” she says.

Those two doses of Narcan were enough to save the child’s life.

On another page, a woman, six months pregnant, two doses of Narcan. She also survived.

In Kensington, drugs are cheap and plentiful, and people shoot up in the open. As she walks the neighbourhood, Ms Pichardo finds people passed out on the pavement, a woman in a stupor with her trousers down, a man lying prone next to a metro turnstile, another man in a wheelchair, his eyes closed and money in his hands.

He, like a growing number of opioid users, has had a limb amputated. A new drug on the street, the animal tranquilliser Xylazine, is being mixed with fentanyl. It leads to open wounds which become infected. The air is rank in places.

John White is 56 years old, and for 40 of those years he has struggled with addiction. At Sunshine House, Ms Pichardo serves him a bowl of homemade soup.

“I’ve been in this city all my life,” he says. “The fentanyl and opioid epidemic is the worst I’ve ever seen. Fentanyl will get you so hooked that you have to get more. So they put it in everything.”

Mr White had a fentanyl overdose after smoking a joint laced with the drug: it is being added to all kinds of illicit drugs, including heroin, cocaine and marijuana.

Ms Pichardo holds out little hope that even if the fentanyl trade is cut off from Mexico that it will improve people’s lives in Kensington.

“The problem that we have with the war on drugs is – it didn’t work then [and] I don’t believe it’s going to work now,” she explains.

When the supply of one drug is cut off, another replaces it, she says. “Once there was heroin, now there’s no more. Now there’s fentanyl. When there’s no fentanyl, now it’s going to be Xylazine. So it’s like they’ll find a way to keep people addicted so that people can make money off of people, off the suffering of people,” says Ms Pichardo.

Directly across from Sunshine House, a young woman is found collapsed on the pavement, her body splayed across the concrete: she’s unresponsive. Ms Pichardo is quickly on the scene, her medical kit by her side, yet again administering Naloxone. The woman is eventually revived – she will survive.

Roz Pichardo returns to Sunshine House, another life saved and another digit to be added to the back pages of her tattered Bible.

Top picture: Darren Conway, BBC