After a turbulent upbringing in which he had lived in 20 different children's homes by the age of 15, he took to heroin in the Eighties, when Edinburgh was threatening to become the Aids capital of Britain.
He was injecting the drug every day and turned to crime to raise the hundreds of pounds a week that his habit cost him. He said: 'I had what junkies call a needle fixation. I had one foot in the grave.'
John, 29, is still an addict: every day he gets 55mg of methadone, a heroin substitute, on prescription from his general practitioner to take orally. But he has stopped injecting, stopped stealing and started to regain his self-respect.
He is one of 1,200 addicts in the Scottish capital who now get drugs such as methadone, dihydrocodeine and diazepam prescribed by their family doctors. The scheme has been running for five years and the arguments about it remain fierce.
A new report suggests that the CDPS has achieved a dramatic reduction in the number of addicts injecting. Now only about a quarter take drugs this way compared with almost 90 per cent when the scheme started.
The programme has also reduced the spread of HIV through sharing infected needles and supporters of the CDPS believe it has contributed to a falling crime rate. Its opponents, however, say it has created a new cycle of dependency.
Addicts are referred to the CDPS by GPs and assessed by a team led by Dr Judy Greenwood, a consultant psychiatrist. If they are given prescriptions, the administration of drugs is supervised by the service for three weeks. Then family doctors take over prescribing while the CDPS monitors them. The aim is to stabilise drug use, reduce dosages and gradually get people off them altogether.
There is not much evidence of the latter happening. Dr Greenwood admits that only 40 to 50 of the 2,500 people referred to the CDPS since it began are now free of drugs. But in a report just published by the Centre for HIV/Aids and Drugs Studies, Sally Haw, an independent drugs researcher, said that only 24 per cent of Edinburgh's addicts had injected in the past six months. Ms Haw says that 'the reported shift from injecting to oral drug use . . . has helped contain the epidemic of HIV infection among Edinburgh drug users, at least in the short term'.
Crime in the region has dropped by 8 per cent since last year and the CDPS believes that reducing addicts' need to steal to pay for drugs has been a factor.
However, Ms Haw's survey discovered that most addicts misuse their prescription in some way and that 89 per cent had used street drugs in the past six months.
Detective Chief Inspector Jinty Kerr, head of the Lothian and Border Police drugs squad, supports the scheme. She said: 'Heroin has not disappeared from the streets of Edinburgh but there is much less of it.'
But some doctors remain opposed. Dr Ian McKee, a GP, said: 'What you are doing is putting a large amount of prescribed drugs into the community. There is a torrent leaking on to the black market.'