He had been good at sports, but his mother insisted he focus on his studies. Discouraged, he took to the streets, where he had his first taste of heroin.

'I gave it a try because I thought I could control myself. I felt so relaxed, and then I forgot everything,' Cheng says.

For eight years, he took heroin every day. In an effort to stop, he and other addicted friends tried locking themselves in a flat. They bound the door with iron chains and threw the key out the window. But the next day they forced their way out and got high.

The deeper Cheng's addiction, the more illegal and immoral his acts became. He worked finding prostitutes, using the money to buy drugs. On and off, he was arrested by police four times on charges of drug possession and trafficking.

'My mum was always crying ... But she never gave up on me in those years.'

Cheng was sent to a government-run correctional treatment centre on the remote island of Hei Ling Chau, but he did not succeed in quitting heroin.

'Inmates describe the life there as 'counting down days',' Cheng says. 'No one viewed it as a place to treat addiction.'

Cheng was finally able to turn a corner after realising how much of his past he regretted and how much of that past was tied to heroin.

'I always remember, when I was just 16 and my dad first came to Hei Ling Chau. He had two big bags of things with him he wanted to give me, but security would not allow him in. I saw him taking the bulky bags back, standing at the ferry pier alone.'

Now, Cheng is a counsellor working part-time for the Society for the Aid and Rehabilitation of Drug Abusers, a group that runs the treatment centre that saved him several years ago.

'I want to be like him,' said one addict who is receiving rehabilitation services. 'Dennis is my role model.'

When asked why so many teenagers were taking drugs, Cheng cites society's emphasis on a knowledge-based economy, which drives less confident young people to take up drugs to escape reality.

'If a young person is marginalised at school, it's very likely they will get in touch with the periphery of society - the triad societies,' Cheng says.

He says the government should broaden the education system to allow young people to find out where their interests lie.

Cheng's boss, superintendent Bill Lee Kwok-biu, described him as a rare case.

'It's no doubt our responsibility, after saving his life, is to offer him a full-time job upon his graduation at Polytechnic University,' Lee says.

Both Lee and his colleague Mary Yip Chai-fan agree that most people in Hong Kong have a flawed understanding of drug abusers.

'People would equate drug users with criminals, but many of them do have regrets and feel pain,' says Yip, a social worker. 'They just can't control themselves.'

Cheng considers himself lucky and says he now understands the cost of his previous actions.

'If I take drugs again, I will lose everything,' he says. 'I wouldn't have started taking heroin if it were 19 years ago now.'