Adrian Thomas has been taking drugs for more than 20 years. He started when he was at university, moving swiftly on to heroin, and had a "vast habit" by the time he left. Since that time, he says, he has been clean for no more than four weeks.

In the 1980s he used to buy drugs from bikers in the Balham area of south London where he lived. "I knew quite a few dealers. Sometimes I had to go to some desperado down a block of flats in the Elephant and Castle. That was desperate."

He was taking heroin with amphetamine, which meant that instead of sitting around blissed out with his eyeballs rolled back in his head, he got up and did things. He worked in computers for an IT company and was often to be found collapsed, dribbling over his desk, after working a straight 24 hours. "Using drugs enables me to work. To be honest, I am not that interested in computers. But when I get hold of a problem I find I can sort it out."

A decade ago he found his drug habit was getting out of hand. "I was using a gram and a half of smack a day. It was pretty horrendous. I tried to cut down. I couldn't do it."

A computer company he worked for at the time paid £15,000 for him to be treated in a rehabilitation clinic, but it did not work.

It was then that he was put in touch with Stapleford Centre, a clinic that treats addicts. It tried him on oral methadone at first, but he couldn't get on with it. Now, each day he has 1.6 grams of morphine sulphate - a heroin equivalent - and 30 milligrams of Dexedrine, an amphetamine.

Today, aged 42, he has been maintained on the same prescription, holding down his job and leading a stable life, for the past 10 years. He lives with his mother.

All his employers have known about his condition and have accepted it, he says.

"The thing I hated about oral methadone was that I felt so flat. I didn't feel anything. But with this [morphine plus amphetamine] you get enough of a buzz not to need to go out and score. I haven't scored for years and years."

If the Stapleford Centre were to close, he says, he doesn't know what he would do. "I have nightmares about that. I wouldn't be able to do anything. I wouldn't be able to stand up."