To understand the war on drugs in the Americas you have to understand the economics, according to Sheila Bond. "You have kids who make $2000 a year and you offer them $500 to take someone out. They don't care if they take out six other people. They're getting a lot of money. The whole thing comes down to the financial aspect. Always."

Bond has seen the war on drugs from the inside. A former Drugs Enforcement Agency (DEA) officer with two decades of experience, she has faced up to Mexican gangsters carrying machetes, gone undercover to trap dealers and worked the investigation of the murder of Enrique Camarena, an undercover DEA officer who was kidnapped and tortured by a Mexican drug cartel in 1985. Though retired, she offers the DEA's line on the ongoing attempt to stem the flood of narcotics into the US.

She retired 12 years ago but every time she speaks about the DEA she still says "we". When it comes to the war on drugs it's clear which side she is on. "It's the war that I still fight to this day and it's personal to me," she says.

Bond, who now lives in South Carolina could be the real-life equivalent of Emily Blunt's character in Sicario, Denis Villeneuve's dark, doomy thriller about the fight against Mexican drug cartels. Not that she'd thank you for that description. "Well for starters [the film] is more FBI than the DEA and that's a major difference, because we feel the FBI should not be involved in drugs. That's our business. But as far as everything else – the Mexican cartels, the drug dealers, the [smuggling] tunnels – the film was very accurate in many regards. A lot of people are worried about putting up fences. What about the tunnels?"

The film is a disturbing take on the ongoing murderous violence of the Mexican drug cartels, told from a North American perspective. So much so, that the Mayor of Juarez called for residents to boycott it on its release in the cinema last year.

But it's clear that the film is concerned with an unfinished story. Despite the recapture of Joaquin "El Chapo" Guzman, head of the biggest drug cartel in Mexico earlier this month, it's unlikely that the flow of drugs across the US border is likely to dry up. Gusman's Sinaloa cartel only came to prominence in 2005 when the DEA managed to capture Javier Arellano, boss of Mexican drug cartel, the Arellano Felix Organization (AFO), infamous for beating victims to death with baseball bats as if they were a pinata. Instead of cutting supply with the arrest, however, it merely cleared the field for others to take over.

But Bond, who is supporting Republican candidate Ben Carson in the current Presidential race for the White House, remains convinced that the war on drugs is one that must be fought. "A lot of people say 'do you think you are succeeding in the war on drugs?'" Bond says. "It's a big war. You take down one battle at a time. So we may take something down and score a hundred kilos of cocaine; 10,000 are coming in. We did our part and we got a little bit. The business is supply and demand."

And it's one that changes course when necessary. "We stopped exporting ether to South America so they have started throwing in other chemicals. If regular cocaine is made with benzene that's a carcinogen so if you try this stuff you're going to kill yourself."

Bond joined the DEA after a stint as a New York police officer. Assigned to Houston she became a gang expert. "Most of my work was on motorcycle gangs and, of course, women don't go under cover in motorcycle gangs because they're just a piece of property. They make their money on titty bars and selling metamphetamine. Then I went to LA and worked on a lot of undercover things. Heavy duty."

It was Bond's role to set up meetings with dealers, exchange money for drugs leading to arrests. There was inevitably an element of risk about it. "I had a Mexican who had a machete. He was in the tree business. But I would feel very comfortable to do a deal with a gun. Even if they patted me down and found the gun or asked if I was carrying a gun I would say 'you bet your ass I do.' And they'd say 'why?' And I'd say 'I'm 5ft2 and there are three of you mopes. What am I going to do?' You have to be able to explain yourself."

Bond's description of life in the DEA takes in bugbears typical of any large organisation. Interagency rivalry is a fact of life. She is amusingly scathing about the FBI ("I've taken a lot of FBI classes. They're good at giving classes, not much good at anything else."). And even the DEA have faced budgetary squeezes. Experienced agents eligible for retirement are encouraged to do so, she says "because then they could hire two or three agents for the same cost."

Three and a half years ago Bond was the victim of an assault that left her with some brain damage. "I'll never be fine, but I'm alive," she admits. She has retained a cutting sense of humour, however. She is against the decriminalisation of marijuana. "Marijuana is a hallucinogen so it does work on your brain and it kills your brain cells. I have the evidence of that in my family because my sister is an idiot. She's smoked all her life."

But in a way that flags up the problem that the organisation she used to work for faces. The demand for drugs is not going away. "Once we get a real President we have to put up a fence and we have to get our military or more border patrol people down at the border," she argues.

That still won't solve the problem in Mexico where American demand has fuelled the rise of the cartels of course. "We're trying. We have officers working there. Unfortunately most of the military is corrupt and most of their police departments are corrupt. I personally have taken six families of Mexican police officers and given them status in the United States because they've come forward and worked for us. But high-ranking officials do have a tendency to get killed. They have to know the danger that exists for their position."

The war goes on. There's no sign of an end to it.

Sicario is available on Blu-ray, DVD, and digital platforms from Monday, courtesy of Lionsgate Home Entertainment.