It's a quiet morning in a quiet road just off Mare Street in Hackney, east London. Drop-in hours are not until the afternoon, but already people are arriving at the doors of Addaction Hackney Community Drug Service, one of Addaction's 70-plus British treatment centres - there's even one in the shadow of Manchester prison, poised to catch the newly free when the pull of old haunts becomes almost irresistible.

The centres see about 25,000 people a year; in Hackney they are currently working with about 500. A team of 44 offer everything from tea and advice to pregnancy testing, an emergency needle exchange, mental health assessments, support for family and friends, money-management courses, parenting skills and - a new service this - women-only sessions, because the concerns of female drug users are often very different from those of the men. One strong worry is that their children might be taken into care by social services.

"We take each person on an individual basis, day by day." says Dee Riley, one of the centre's managers. Much of the work involves trying to undo the mental habits of a lifetime. "Some of them don't think they can do anything. They're in their late 30s or 40s and they've never worked. So what we say is, 'You have got skills, you have got abilities.' [Often] all they know is their drug habit. But we say, 'Look, you get up every morning to find your dealer. So you can get up every morning to go to work. And they go, 'Oh.' Some of them are really good at communication, because they have to wheel and deal to get their drugs. And we take that and say, 'Look, you're good at this' - and build their confidence so they can take that positive element into the workforce. Or they're good at supporting people ... We recognise the positive. And when you give that to people they're quite amazed, you know. You see them leaving, walking quite tall."

The ideal, of course, is to get addicts off drugs, and keep them off, but relapse rates are notoriously high. While, as Richard McKendrick, regional director for Addaction South, puts it, "it would be nice to think there was an end-point", this is elusive. Some fare better than others, but as Darren Blair, who was on one drug or another for 24 years and now works for Addaction, puts it, "I don't know what's going to happen tomorrow." Temptation never entirely drops out of sight.

Antonio's story

It has been 46 days since Antonio's last hit, and he looks wary and a bit defiant, but soon his low voice and downcast eyes give way to fluid storytelling. Now 41, he grew up in Birmingham, the youngest son in a stable family of first-generation immigrants from the Caribbean. At the beginning they were the only black family in a white street; neighbours were racist, as were the police. Emulating his elder brothers, he became a "bit of a tearaway, a bit rebellious".

He bought his first cannabis at 12, didn't like it much, "but everybody else did it". It was the 70s and Rastafarianism was in the ascendant. "Black people kind of found a voice and were hanging with that." Racism from his teachers, he says, was subtle, but destructive: he was prevented from sitting exams at 13 - it was simply assumed he wouldn't pass. He started truanting and left school as soon as he could, at 16, to get a job. At 20 he escaped life at home, for London: "Bright lights, big city."

He found work as a West End doorman. Part of his job was to confiscate drugs at raves. He had no interest in ecstasy or acid but tried cocaine when he was about 21. It didn't make much impression. It wasn't until a few years later that he tried it again, with an elder brother, and soon he came to love how it made him feel. But then the lines got fatter, the coke kept aside for the morning began to disappear the night before. He had a partner and a small daughter, but "I stopped coming home, because she was boring and the baby was crying". He never had to turn to crime to get drugs - dealers often pay bouncers a cut of takings, or in kind. He knew he was in trouble when he started finding himself at strangers' houses at dawn, jabbering and paranoid.

He went to Cocaine Anonymous and Narcotics Anonymous, and in 2001 began 14 drug-free months. Then he thought he could cope with the odd line. To break free again took a year. This time he lasted for 20 months, even though his mother died in that time and he and his partner of 11 years split up.

That was when he came to Addaction. "When people like him come to us," says Riley, "it's because they want to be heard. They want someone to help them through the recovery process." Realising Antonio had no formal qualifications at all, case workers guided him on to numeracy and literacy courses; he finally gained the qualifications he had been denied as a boy. He joined an ethnic minority forum, and began acting as an advocate for minorities who find themselves ill-served by the services they look to for help. The break with his partner meant he had to move out of his home; Addaction helped him find a flat.

Still, doorman's work was all he knew, so he went back, resisting temptation until early one morning, when "I thought, 'I just want to have a look at it, just smell it' - next thing I knew I was off again." One thing he wants to make clear is how much he loves the stuff. No amount of knowledge about the damage changes that: "I love to play with it and move it around and wallow in it like an angel."

So he found himself at Addaction again, this time with a very specific aim - "to get into a different way of life", give up the night-time existence, work nine-to-five like everybody else. "I've always wanted to get educated to a higher level. I want to do poetry and music and creative writing." Fitness, as well. "When you're running hard and sweating - you just feel brand new."

Darren's story

These days, says Darren Blair, "I get people ringing me up and saying, 'Thanks Darren, you saved my life, you changed my life.' That's what my day's about."

Blair is a boyish 40-year-old Londoner, a soft-voiced, gap-toothed charmer. He was put into care at four weeks old, and dates his problems from the moment, aged four or five, when he realised both this, and the fact that he is black. He had been placed, as he often was, with a white foster family, and sent to an all-white school, where he was bullied -a pattern repeated again and again. "There was always racism," he says, compounded with what he now sees, with the obvious benefit of therapy, "as the frustration of not having no identity". He would walk past black people in the street "and think, 'That could be my mum, that could be my dad'". He developed involuntary nosebleeds, and his jaw locked up from grinding his teeth, for which he had to be sedated at night. At eight or nine he began to act up every time he was placed with a new family. "God bless them, there wasn't a bad person there, there were kind hearts and everything", but "after six or seven weeks, I'd just have a violent fit, smash up the place".

He was moved to more than 20 different families, and placed in a children's home when he was about 12. When, at 16, he finally met his mother, he swore at her with all of a decade and a half's-worth of anger and hurt, and then walked out of the room. He has a better relationship with her now, but it has taken years. "I don't have any children of my own, and I don't think that's a coincidence."

At 12 he also had his first joint, and found it calmed him; then he found that dealing small amounts at school was a good way to bolster his £2.70 allowance, and make friends. "If I couldn't pick my family, I could pick my friends, and my friends became my family." On his 18th birthday he had his first line of cocaine, "which I loved" because it masked insecurity; "within 48 hours I was freebasing". By the time he was 25, working at Camden council, he needed both alcohol and coke every day. One year he took 156 days off. At 28, he quit. A severance package let him travel the world; everywhere, from America to Asia, he says, he'd score within 45 minutes of landing.

Back in England, he sank into depression. Twice he tried to commit suicide. After an overdose, he lay on the floor for seven days before anyone found him. Narcotics Anonymous helped him get clean for a year, but he relapsed spectacularly with crack and heroin.

Relationships were impossible. "The only relationships you have with women are sexual ones with other users." He turned to robbery, mugging and shoplifting, for which he finally got caught. He chose prison over bail, hoping to get off drugs. But prison was awash with heroin. It took another five years to come clean. It was residential detox that "changed everything. Since I did that I haven't used." That was four years ago.

"He wanted to give back the help he had, to support others," says Riley. Blair applied for a place on a Smart scheme, which trains people to work in the field of addiction. Addaction Hackney hired him - it does this only if a user has been drug free for at least two years - and he now works mostly with people whom the courts have decided can be dealt with in the community rather than in prison, though some have arrived voluntarily. With a colleague he leads groups dedicated to relapse prevention (helping people to identify triggers that have made them resume drugs, how to avoid or neutralise specific vulnerabilities); and harm minimisation (how to inject safely, how to avoid ODs, why users shouldn't share any paraphernalia at all, how to stay as healthy as possible). He sponsors people in Cocaine Anonymous, is going to college, doing NVQs, watches films in his spare time. "There was a stage in my life where I thought death was the only option for me."

· Some names have been changed.