Spliffs aren’t what they used to be. In the late Sixties and early Seventies when I was puffing my way through a wayward youth, dope was pure and mellow, man. In those golden if rather hazy times, a joint made you giggly, peckish and jiggy. It was fun and we were cool. Or at least we thought so.

Cannabis today is rather different. While some varieties approximate to the stuff of yesteryear, newer strains introduced over the past 10 years have been genetically modified to produce a more intense high with double or treble the strength of the ordinary stuff, according to the UK’s Forensic Science Service, which regularly analyses drug seizures. This super strong cannabis, commonly referred to as “skunk”, is, in some areas, more readily available to children and young people than the weaker varieties. And it’s not just a weekend party habit; strong as it might be, skunk is smoked before and after school, even during break times.

The debate over cannabis has come to the fore with the announcement early last month that the Government intends to review the classification of the drug. In January 2004 cannabis was downgraded from a class B drug to class C, placing it alongside steroids and some prescription antidepressants. But recent studies have indicated that cannabis use increases the likelihood of developing mental illness.

While skunk is worrying in itself because of its potency, there is a growing body of evidence to show that the effects of habitual smoking of all types of cannabis, particularly among young people, can reduce motivation, impede short-term memory and increase susceptibility to psychosis and schizophrenia.

Marjorie Wallace, chief executive of the mental health charity Sane, has been campaigning for 18 years around the negative impact of cannabis on the minds of people at risk of mental illness. “Eighty per cent of patients in acute wards of psychiatric hospitals presenting with schizophrenia have a history of taking strong cannabis,” she says. “Young people who think of it as a recreational activity don’t realise that they could be harming themselves for life.”

In a column for TES Friday magazine (February 4), consultant psychiatrist Raj Persaud wrote: “If you have tried cannabis relatively frequently, just 50 times in your life, your chances of developing schizophrenia are increased sixfold.”

Possibly the most damning research was carried out by Robin Murray, professor of psychiatry at the Maudsley Hospital in south London. “We found evidence to show that if a person starts smoking cannabis at 15, they are twice as likely to develop psychosis and that the earlier they start, the more dependent they are likely to become,” he says.

“We also found that people with mental health problems or with a family history of it were more likely to develop psychosis if they smoked cannabis than if they didn’t. While the vast majority of people who smoke it don’t come to any harm, it is clear that using huge amounts over a long period of time, particularly if you’re young, increases the risk of schizophrenia and psychotic disease.”

Professor Murray’s work did not distinguish between skunk and the other varieties. All cannabis, in his view, has the potential to cause serious psychiatric disorders if taken regularly by young people who either have a family history of it or who are vulnerable to mental illness.

But it is skunk in particular that has become the focus for concern.

“Strong cannabis is nothing to be played around with,” says Colin Stewart of Release, the drugs advice and information service. “Although we have to be careful not to get hysterical about it, the fact is that this stuff does induce psychosis, not just in people with the experience or history of it, but in those with a potential for it - which you can’t know until it happens. If a kid is going through emotional problems, such as a relationship break-up, skunk will make it worse. It’s something people have to be aware of.”

There is no doubt in the mind of Camila Batmanghelidjh, child psychotherapist and director of Kids Company in Camberwell, south London, that some of her extremely vulnerable young clients have suffered profound damage from smoking strong weed. “We have kids who’ve had psychotic breakdowns because of it,” she says. “We don’t know what’s in it, but we do know that skunk is more available to kids in poor neighbourhoods than ordinary cannabis. And because it’s available, they use it habitually to numb themselves or to achieve an exaggerated euphoric release to escape their misery.”

A recent British Crime Survey report shows that there has been a drop in the number of 16 to 24-year-olds using cannabis to 24.8 per cent in 2003-04, from 25.8 per cent the previous year and 28.2 per cent in 1998.

How they use it is another matter. The UK Independent Drug Monitoring Unit report, Cannabis Use in Britain, reveals that young people who smoke do so habitually: between one and six spliffs a day, though up to 20 is not uncommon. They use and think of cannabis as they do tobacco and caffeine; just over 48 per cent of respondents said they smoked daily.

The effect of daily use isn’t known, but Susan Greenfield, Oxford professor in synaptic pharmacology, has been a vocal opponent of the Government’s reclassification of cannabis to class C because of her concern for what she has called the “severe impairment in attention span and cognitive performance” among regular smokers, even after they have given up, as well as for its links with psychosis. She cites laboratory experiments that indicate shifts in the pattern of networks that connect brain cells, thereby causing personality changes. One common change is the way it robs people of their motivation. And for teachers, that is evidence enough that we have a problem on our hands.