The programme is run by Calton Athletic, a self-help group funded by Greater Glasgow Health Board and Glasgow City Council which advised the makers of the film Trainspotting. In four years it has reached thousands of children with personal accounts of how drug abuse can wreck lives.

But a Scottish Office study carried out by the Centre for Drugs Misuse Research at Glasgow University has expressed concern. One official commented: “I have major reservations about people standing up in an evangelical way and explaining how bad they have been.” A second said: “It was another night of Dracula.”

David Main, Calton’s deputy director, said: “Some call it shock-horror but we call it reality. We tell kids about the life of an addict and the effect addiction has on their family.”

Pupils and parents whose views are reported are more positive. They say the workers are “very credible and knowlegeable as a result of their own personal experiences”.

These views run counter to the Government’s drugs education policy which argues that frightening young people may increase interest and experimentation. The Government also urges schools to be cautious about inviting in former drug users. In contrast, 96 per cent of the 65 parents in the Scottish survey were in favour. The same parents take a dim view of “harm reduction” tactics, now the official line in drugs education, which accept that young people will experiment and so they should be helped to minimise the dangers. Only 12 per cent believed the approach should be used in schools.

Calton is adamant about “zero tolerance” and says it is a myth that taking cannabis will not lead to hard drugs. “No one involved in our organisation started out to become a drug addict. Everyone tried to use drugs safely, ” Mr Main said.