Heroin abuse in the US has been spreading beyond inner cities, resulting in a sharp rise in addiction and death. Chicago is a hub for cheap, pure and plentiful heroin, much of it supplied by Mexican drug cartels.
Chicago's "L" train green line leads directly to the open-air drug markets on the city's west side.
As we travel the route with one of the addicts, Jason, he phones his contact. He wants two bags of heroin, each costing just $10 (£6). The dealer meets us, and within seconds two tiny bags are handed over.
This part of Chicago has been ground down by neglect, drugs and crime, and residents talk openly about the narcotics on sale.
Of the four people who stopped to ask what we were filming, all said they had taken heroin.
The police are here, but they seem to face daunting odds as the heroin abuse spreads.
Nearly half a million Americans are thought to be addicted to heroin. One woman we meet in a county jail - she was locked up for stealing to feed her habit - calls it an "epidemic".
"I don't think these police officers know how bad it is out there - I really don't," she says.
Back on Lower Wacker Drive, home to central Chicago's destitute for nearly a century, five heroin addicts are injecting in an underpass.
Some bleed as they repeatedly stab the needle in - desperately trying to force the light brown fluid into their bodies.
Greg can't find a vein and injects straight into his bicep - what he calls "muscling it".
"My arms are fried ," he says. "It sucks. This is what I have to do nine out of 10 times is muscle it because my arms are so trashed."
Greg and Stacey are sleeping rough.
Stacey first took heroin when she was just 11. Now the two of them live like husband and wife in this subterranean netherworld, partners, addicts and the parents of three young boys.
"The hardest thing is just not being there for them," says Greg. "I think about them every day. I try to just numb it with this dope but it's just hard, man."
Much of the heroin supply comes from Mexico, where production has risen more than 600% in the last 10 years.
Heroin is often cheaper and easier to use than prescription drugs, some of which have become more expensive, harder to obtain and harder to abuse - they now come in versions that are not so easy to grind down to snort or dissolve.
Last month Mexican authorities arrested Joaquin "El Chapo" Guzman, the suspected boss of the Sinaloa cartel.
The Chicago Crime Commission (CCC) named him last year as the city's Public Enemy Number One.
The Drug Enforcement Agency (DEA) estimates that the cartel supplies as much as 70% of the illegal drugs sold and used on Chicago's streets.
But it's not clear that the arrest will help stem the flow of heroin, especially if demand in the US remains strong.
Increasingly, it's not just the inner-city junkies who are using heroin.
The recent death of actor Philip Seymour Hoffman from an accidental overdose of a mix of drugs including heroin drew attention to something the police have known for a while - heroin now crosses all boundaries.
"Heroin addiction is probably at its all-time high," says Special Agent Jack Riley, the DEA's regional head.
"I've been doing this for 30 years in virtually every corner of this country and if anything can be likened to a weapon of mass destruction on a family, on a community, on society, it's heroin.
"I just don't understand why people across the board don't see its danger. Social services are overwhelmed, our healthcare services are overwhelmed, yet Mexican organised crime and street gangs make billions from it."
The biggest increase in users is among the young.
Research suggests that nearly 34,000 12-17 year olds are now trying heroin for the first time each year, as the drug becomes cheaper and more readily available than ever.
Many live beyond inner cities, in small towns or in the country.
Steven Lunardi was raised in the middle-class suburbs that surround Chicago.
He started using heroin when he was 18, and says that "everything changed" when he started injecting it.
"My health, appearance, friends, hobbies and belongings all began to wither away," he says.
He used to skip university to buy drugs on the west side, injecting in between classes in the bathroom.
He says: "During my worst times I was spending a couple of hundred dollars a day, approximately," - about 25 bags.
After going through rehab, Lunardi has been clean for more than a year.
But not all users find a way out.
Stephanie Chiakas was just 17 when she died of an overdose. Her father, Ken, says he cries almost every day.
"I'm not looking for sympathy or anything," he says, "it's just something I can't control. The feeling is unbelievable."
Stephanie's family made a film in her honour. Pictures of her dissolve in and out, first as a little girl, later as an all-American teenager dressed for prom night.
"I just want to speak out and educate people," says Mr Chiakas.
"They don't want to talk about it. They don't want to believe it.
"It's not the picture anymore of someone living under a bridge shooting up with needles.
"It's your next-door neighbour. It could be your kid."