The scene which greeted Tijuana's paramedics as they entered 'La Perla' bar in the early hours of the morning was grim.

Two men were unconscious - a heavy-set man sprawled on the floor, his friend slumped in a chair - both clinging to life by a thread.

Once more, the city's emergency services had been called out following a suspected fentanyl overdose - increasingly part of every nightshift, says paramedic Gabriel Valladares.

"It's getting worse. We're seeing more and more, and it's always fentanyl," he says.

The synthetic opioid is 50 times stronger than heroin and is making the paramedics' job much harder.

"We generally see two or three overdoses a night. But we've had as many as six or seven cases in a single call - probably because they all took the same substance," adds Gabriel.

Some in the team quickly began CPR on the two patients while others prepared doses of Narcan, the most effective drug to reverse a fentanyl overdose.

The two men may not have even known they were taking fentanyl. Because the opioid is cheap and easy to produce and transport, Mexican drug cartels have begun to cut it into recreational drugs like cocaine.

The Mexican border city finds itself in the grip of a full-blown drug epidemic. But the country's president, Andrés Manuel Lopez Obrador, has played down the extent of the problem.

"We don't produce fentanyl here. We don't consume fentanyl here," he said last year. Following that controversial claim, he has promised to introduce new legislation to Congress to ban the consumption of fentanyl and other synthetic opioids.

Those working on Tijuana's frontlines fear that may be too little, too late.

The director of the state's forensic services, Dr César González Vaca, tells me that for over a year his department has tested every dead body that comes into their morgues in two border towns, Mexicali and Tijuana, for fentanyl.

The study has shown that around one-in-four bodies in Mexicali contained fentanyl, he says, and last July, the statistics for Tijuana were as high as one-in-three.

"It seems the closer we are to the border, the more consumption of this drug we see", explains Dr González Vaca. "Unfortunately, we can't compare to other states in the Republic as, in Baja California, we're the first state to carry out this study," he adds, urging his counterparts around the country to help build a clearer national picture.

People working with the living in Tijuana also claim the president has underestimated the scale of the crisis in Mexico.

Prevencasa is a harm reduction centre in the city which provides a needle exchange and medical services to addicts. Its director, Lily Pacheco, randomly selects two used needles and two empty drug vials from their disposal unit.

All four items of drug paraphernalia test positive for fentanyl. The city is awash with it, says Lily.

"Of course fentanyl exists. To suggest otherwise is a lack of recognition of this reality. We have the evidence right here," she says, pointing at the testing strips.

"The overdoses we see and all those who've died from fentanyl are part of that evidence too. Ignoring the problem won't solve it. On the contrary, people will keep dying."

As our interview ends, there is suddenly a much more visceral illustration of the crisis than fentanyl tests on used syringes.

Lily is rushed outside where someone is overdosing on the street. She carries Narcan too, donated by a US charity after her federal funding was cut, and saves the man's life.

He was lucky. But many were not so fortunate.

The fentanyl epidemic has hit the neighbouring US - the world's biggest market for illegal drugs - especially hard. There, an estimated 70,000 people died of overdoses last year.

Elijah Gonzales was one of them.

Just 15 when he accidentally overdosed on a counterfeit Xanax pill from Mexico, he had no idea it was fentanyl-laced. Text messages Elijah's mother, Nellie Morales, found afterwards suggest it was his first time experimenting with drugs.

His body simply couldn't cope.

"I miss him every day," says Nellie in her apartment in El Paso, Texas, adorned with pictures of her son. "He was going to graduate this June. A piece of me died that day that he died."

Unfortunately, such deaths are common in the US. More than five Texans die every day from fentanyl, say state authorities, and in El Paso County alone, fentanyl was involved in 85% of accidental overdoses like Elijah's.

City police compare the situation to the crack epidemic of the 1980s.

El Paso sits across the border from one of Mexico's most dangerous cities, Ciudad Juárez. When we visited, US customs officers seized 33kg (73lb) of fentanyl in a single day, enough to kill everyone in El Paso twice over.

Arguments over the drug have even seen some Republicans advocate for sending troops into Mexico to fight the cartels. No doubt such debates will feature highly in the US election campaign. In truth though, given how easily it can be transported, it is almost impossible to stem the flow of fentanyl into the US.

In Ciudad Juárez, I meet Kevin - not his real name - a 17-year-old drug smuggler and hitman for La Empresa cartel. He shows me videos of his gang moving the drug through tunnels beneath the US-Mexico border.

"A kilo of fentanyl makes the cartel around $200,000 (£160,000) in the US", he says, "I earn about $1,000 (£800) to take it north."

Kevin has been working with the cartel since he was just nine. But he has never seen anything like fentanyl. He predicts it is the future of the illegal drug trade:

"It's the strongest drug I've ever seen, chemically so powerful that people keep demanding more and more. It's going to keep blowing up," he says.

I asked him if he felt any remorse over the deaths of US teens like Elijah.

"No, it's all part of a chain", he shrugs. "They send guns south, we send fentanyl north. Everyone's responsible for their own acts."

Back in Tijuana, it took three doses of Narcan, but the paramedics managed to bring one patient back from the brink in the 'La Perla' bar.

For his friend, though, it was too late. He died amid the beer bottles and empty glasses on the barroom floor.

The paramedics' dignified silence is pierced by the awful sound of wailing. His mother has made it to the bar only to be told her son, at 27, is another victim of this most powerful of narcotics, his death a footnote in an election year on both sides of the US-Mexico border.