The full extent of the effects of cannabis on the body and behaviour continues to be researched, and there are many mysteries still be explained about just how the drug affects the brain the way it does.

In the meantime, some of the world's top scientists have
already predicted that by 2010 almost a quarter of new cases of schizophrenia in young people will stem from cannabis
smoking.

Anecdotal evidence arising from court cases relating to burglary, arson and violent attacks where perpetrators were known cannabis users, seems to illustrate the potential the drug has to alter personality, trigger psychotic behaviour or exacerbate existing mental illness, and should leave us in no doubt that cannabis ought not be viewed simply as a mild recreational toy.

The medical journal The Lancet recently reported that any use of cannabis increases the risk of psychotic illness by 40 per cent. Regular use, if only once a week, can increase the chances of such mental illness by as much as 200 per cent.

Studies have found that more schoolchildren in England have taken cannabis than in any other European country. Prof Colin Blakemore, chief of the Medical Research Council, says the link between cannabis and psychosis is "quite clear".

Statistics from the NHS National Treatment Agency show that the number of young people in treatment for cannabis psychosis almost doubled from around 5,000 in 2005 to 9,6000 in 2006. Cannabis was downgraded to a Class C drug by then Home Secretary David Blunkett in 2004, and possession of the drug in small quantities was no longer an arrestable offence.

Yet scientists like Professor Neil McKeganey of Glasgow University's Centre for Drug Misuse Research say we are facing a generation "blighted by the effects of cannabis use".

Anyone moved to laugh off the idea that cannabis can lead to, or aggravate, severe personality disorders should consider the case of Ivan Hill, the 31-year-old Rotherham man who plunged to his death from a town centre bridge, while affected by cannabis-induced psychosis.

Police had spent an hour trying to coax Mr Hill down from his perch, during which time a crowd gathered to watch the spectacle, including a few idiots who shouted: "Jump".

A psychiatrist told Mr Hill's inquest that only hours before his death he had discharged himself from a mental health centre. Use of cannabis was said to be the most likely cause of his psychosis, which made him depressed and paranoid.

Teenage killer Luke Mitchell's heavy cannabis use left him unable to distinguish between fantasy and reality. The Scottish 16-year-old's gruesome murder and mutilation of his 14-year-old girlfriend Jodi Jones was said to have been inspired by delusions fuelled by cannabis. The judge in the case said he did not "subscribe to the notion that this is a harmless recreational drug".

In Lincoln, cannabis addict Robert Dickenson, 52, shot dead his neighbour then killed himself after a row over a privet hedge. The coroner said: "There are people who suggest cannabis is a harmless drug. This is not the case. It is a drug that has an effect on the brain, and it certainly had an effect on Robert Dickenson for the worse."

Health professionals working with drug users suffering from mental illness say it's unusual for a patient only to be a user of cannabis. As many families fear, cannabis can be a "gateway" to other, more harmful drugs. Users often continue to take cannabis as well as other substances.

Meanwhile, various lobbying groups and politicians mull over the problem of cannabis abuse and whether reclassifying the drug back to a class B would make any difference to the tide of much stronger cannabis (skunk) now washing over the UK. Skunk is up to 20 times stronger than the cannabis used in the 1970s.

Many professionals working with drug users say public education on the health risks is more important than the substance's legal status.

"Incidence of cannabis-related psychosis has not gone up since the drug was reclassified," says Paul Jenkins, chief executive of mental health charity Rethink.

"We don't think reclassification back to Class B will have much, if any, effect. We're far more keen to see the Government committing money to a national awareness campaign about the health risks associated with cannabis, like the one carried out in France.

"The peak period of incidence of schizophrenia is between the mid-teens and mid-20s in men, and a slightly younger group in girls. It's thought that the younger end of this group is particularly vulnerable because the brain is not quite fully formed."

Cannabis does not only bring on mental illness, it also affects the brain's ability to recover from it. "One perception that is prevalent, particularly among the young, is that cannabis is okay and the much stronger version, skunk, is bad. There are serious risks with both.

"Cannabis intoxication is like alcoholic intoxication," says consultant psychiatrist Duncan Raistrick, a specialist in addictions.

"In small doses, it makes users feel relaxed and gregarious. But as tolerance, use and dosage increase, it can make people feel panicky, anxious and paranoid, as well as lead to hallucinations."

Psychosis, "badness" that means the user loses touch with reality, comes about either as a result of using cannabis, a state from which the user recovers in time, or because the cannabis has triggered or aggravated psychosis in a person who has a genetic predisposition to it or is already mentally ill.

In some cases, cannabis – which takes about two weeks to be eliminated from the fatty tissue in the body – can lead to long-lasting effects like schizophrenia, whose symptoms include extreme paranoia and hearing voices, usually in people already vulnerable to the condition.

It is treated by stopping use of cannabis and by drugs similar to those used to treat psychosis.

Some schizophrenics use cannabis because "it makes the voices go away", but this effect is only temporary. Patients might neglect to take their normal medication while using cannabis, which will interfere with the effectiveness of treatment.

"It's questionable whether reclassification would do any good, and do you change only the classification of the more dangerous version, skunk?" says Dr Raistrick. "Reclassification would also give the police more work, and the prisons are already full, as are addiction services.

"As far as a campaign of public education goes, it would probably be an uphill struggle.

"Cannabis is so much part of the culture now, and we all know that there are thousands upon thousands of people who do take cannabis without any ill effects."