“I have tried smoking cannabis since, but it was terrible. As soon as I took a couple of puffs it made me feel instantly crazy. It was like putting on a switch. I don’t touch the stuff now.”

The voices and hallucinations in schizophrenia result from an excess of the brain chemical dopamine. Drugs such as cannabis, amphetamines and cocaine increase the levels of dopamine in the brain. The Lambeth Early Onset (Leo) service at Lambeth Hospital in south London has a first-onset psychosis ward where the damaging effects of cannabis on dopamine levels are all too easy to see.

Young men and women are brought in as early as possible to improve their chances of recovery with anti-psychotic drugs. In a year, as many as 120 patients pass through the ward — the vast majority admitting to regular cannabis use.

“It would be wrong to say that cannabis alone causes psychosis,” says Paddy Power, a consultant psychiatrist at the unit. “It’s a bit like saying someone had a heart attack because of a stressful incident when they also ate too much fatty food, took too little exercise and smoked. In much the same way, using cannabis can be a major contributory factor in the onset of psychosis. You are also at greater risk of developing psychosis from genetic factors, early brain development problems, birth trauma and even migrating to another country. If you add cannabis, then you have a dangerous mix.

“Between 70 and 80 per cent of the people who present at our unit have a history of cannabis use that has probably been a factor in the development of their psychosis. I wouldn’t say that should prevent a reclassification of the drug, but it would be irresponsible of the Government not to marry that with an educational programme — particularly for parents and young people — so users can be made aware of the risks in much the same way as we are given health warnings over alcohol and tobacco.”

The Home Office says its “Frank” anti-drugs campaign includes advice on cannabis, but its efforts on education timed for the reclassification of the drug on January 29 could be called into question. It has commissioned the mental health charity Mentor to produce a million leaflets for distribution a month after the reclassification — on a budget of just £50,000. Mentor’s chief executive, Eric Carlin, says much more money is needed if the message that all drugs — including cannabis — are potentially dangerous is to get to young people. “The area has been neglected,” he says. “We are not yet clearly making the point that if you have a history of mental illness, or if you are pre-disposed to psychosis, you are playing Russian roulette by smoking cannabis. Our job is to try to get that message across.”

But isn’t reclassifying cannabis from B to C sending a message in itself that the drug is safe? The Home Affairs select committee that recommended the reclassification in May 2002 thinks not, even in the light of the new evidence. David Winnick, one of the MPs on the committee, says its members stand by their recommendation. “We would not change our view,” he says. “I believe we should be warning people that they should not take any drugs, including cannabis. But we decided that to continue to criminalise everyone who takes cannabis would be wrong.

“As opposed as I am to people smoking nicotine, I would not be in favour of banning that. All we can do is warn people of its dangers. There was no evidence to suggest that more people would smoke cannabis simply because we reclassified it, and I don’t believe they will.”

Terry Hammond, Steve’s father, disagrees. “Of course it sends a message,” he says. “After Steve became psychotic I spoke to a lot of his friends about cannabis and they all thought it was perfectly safe — and they felt that the reclassification confirmed that. Since then I have been contacted by lots and lots of parents who believe cannabis was a major factor in their sons and daughters slipping into psychosis. I have no doubt it caused Steve’s.

“Most people who smoke cannabis will probably suffer no harm whatsoever. But for some it will be disastrous. They could sink into Steve’s world. And I wouldn’t wish that on anyone.”

DEBATE

Is cannabis safe?

E-mail debate@thetimes.co.uk