THE street outside Zainab Market in Karachi is a great place for watching people. Traders, customers, students, heroin addicts: everyone has a story. A moment of eye contact can inspire an entire imagined history.

It’s here that I left 26-year-old Hussain. With him there was no need to imagine. We met three weeks earlier when I visited one of the Edhi Foundation’s drug rehabilitation centres. Hussain, all dark skin and scars, was in his sixth day of recovery from an addiction that has plagued him most of his adult life. Like an estimated half a million chronic users in Karachi, Hussain answers to one master alone: heroin.

Hussain told me he checked himself into rehab along with his younger brother Yusaf. He said his addiction had ruined everything, so in an attempt to get his life in order he had summoned up all his strength to drag himself and Yusaf into the centre.“I have a son. He gets angry and calls me a druggy. I have to give up for him.” Yusaf was like Hussain’s shadow. He looked to his brother and followed every hint of guidance. He’d nod when his brother did and often repeat the end of Hussain’s sentences. The day I met Hussain and Yusaf, both of them were going through withdrawal. Their bodies shook as they told me they felt like they were covered in “fire and needles.”

Two days later Hussain and Yusaf decided they were ready to start their new lives. They left the centre and said Edhi had “saved them”. Hussain told me he couldn’t wait to see his wife and son. There was genuine optimism; light in an otherwise murky world of bloody needles and longing.

The Edhi Foundation offers addicts a free rehabilitation service. There are six centres in Karachi with 4,500 patients receiving care at any one time. Treatment is basic. Dr Ayaz, the programme’s lone medic, told me: “We don’t have the resources or the funds for things like methadone. Instead the centre provides a drug-free environment.”

Patients, many of whom are admitted against their will, are locked behind bars. What results is a system of enforced ‘cold turkey’. Sedative injections and paracetamol tablets are offered for pain relief. Those who admit themselves, like Hussain and Yusaf, can leave at any time.

Iron bars and concrete mesh surround a central courtyard for visiting family members. Patients are housed in four distinct sections, marking progressive levels of heroin dependency. As I walked through, the men inside pushed against the bars to tell me they were “completely better now”. One pleaded with me to shake his hand. As I did he gave me a small rosary and asked that I pray for him.

Each time I visited the centre I saw men trying to convince family members that they were ready for release. Staff told me they see a constant stream of patients leave and return within weeks, or sometimes even days. There is little they can do to stop family members taking patients home midway through the withdrawal process.

“Withdrawal makes good people lie. These people will say whatever they need to get out. If they return in a week or a month, we’ll treat them again,” said a staffer. Some patients I met were on their sixth round of treatment.

The scale of the Edhi Foundation’s struggle against heroin addiction in Karachi can perhaps best be digested when driving through the city. One afternoon I asked the Edhi staff if I’d be able to meet addicts on Karachi’s streets. An ambulance driver took me to Esa Colony, then onto Lyari and Malir. I saw groups of men injecting communally, in the middle of the street. They shared needles and looked on as passing children returned from school. They told me their habit is cheap.

The war in Afghanistan has seen poppy cultivation spiral and Karachi’s port allows heroin to be shipped around the world. As a result, heroin in Karachi is readily available and often cheaper than food. All the addicts I spoke to had a shared experience of poverty. Men without work, prospects or hope dive into heroin use to escape. The story was the same every time. One hit leads to a lifetime of dependency, as if the vibrancy of heroin washes out every other colour in life. Any motivation to shape a future becomes secondary to the chase for another hit.

Thinking back to the moment I said goodbye to Hussain, there was complete resignation on his face. It was as if the universe had conspired against him. The war in Afghanistan, Pakistan’s geography, economics and his own history rendered him completely powerless.

Hussain’s return home had not been what he was expecting. His years of heroin abuse had frustrated his wife beyond redemption. She didn’t trust him. Husain told me his wife had asked for a divorce and refused to let him into the family home. She wanted to move on with her life and concentrate on raising her son.

A day later Hussain’s brother Yusaf stole his phone and his last five hundred rupees. Hussain was now sleeping on the street and working his way through Karachi’s soup kitchens in an effort to find his brother. For the first time in his life he was completely alone. The lust for heroin had even rendered brotherly love null and void.

The Edhi system of rehab may not be ideal. Little is known about success rates and the staff and volunteers often seem overwhelmed by the scale of addiction in Karachi. But the foundation offers the only form of support for many addicts in a city that can sometimes feel like it is floating on a sea of heroin.

The documentary Cold Turkey in Karachi will be broadcast on BBC World Service television and radio in mid August. www.mobeenazhar.co.uk Twitter: @Mobeen_Azhar