So how dangerous is skunk?

The UN called the move courageous. Mental health experts described it as a crucial intervention. Sir Richard Branson says it is time to think again. And on pro-marijuana blogs and chatrooms from Sydney to Sarajevo, the mood was more murderous than mellow.

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Seven days after The Independent on Sunday published evidence that almost 10,000 under-18s needed drug rehabilitation for cannabis use in Britain last year, a ferocious debate has begun over whether a potent new form of the drug is leading to growing mental health problems.

Beneath the headline: Cannabis - An Apology, the IoS revealed last week that more than 22,000 people were treated last year by the NHS for addiction and psychological problems caused by smoking skunk, which is up to 10 times stronger than resin or grass.

Ten years after Rosie Boycott, this newspaper's then editor, launched a campaign to have marijuana legalised, the IoS last week reversed its stance, stating that the evidence of mental health problems among smokers made the current 'C' classification correct.

The move has provoked an international debate about the threat posed by cannabis in general and skunk in particular. The warnings about the effects on mental health were underlined yesterday in a new report, saying almost a quarter of all new cases of schizophrenia would stem from cannabis smoking by 2010.

The study, published in the journal Addiction, also predicts that young men who smoke cannabis will be particularly at risk. The use of cannabis among under-18s rose 18-fold in the 30 years to 2002, according to the researchers.

Dr John MacLeod, co-author of the study, said: "If you assume a link [with cannabis] then the number of cases of schizophrenia will increase in line with increased use of the drug." Antonio Maria Costa, the executive director of the United Nations office on drugs and crime, yesterday called for people to wake up to the fact that cannabis is now a dangerous drug wrongly portrayed as a "gentle, harmless substance".

Writing in today's IoS, Mr Costa says that is no longer the case as a result of the potency of skunk, adding: "The cannabis now in circulation is many times more powerful than the weed which today's ageing baby-boomers smoked in college. Evidence of the damage to mental health caused by cannabis use - from loss of concentration to paranoia, aggressiveness and outright psychosis - is mounting and cannot be ignored."

Sir Richard Branson, head of the Virgin Group, who was a supporter of the IoS decriminalisation campaign, said yesterday: "The arrival of genetically engineered skunk has merited a new look at the situation."

Medical experts are now warning that the addictive nature of the drug means that detox clinics are needed. A report published in The Lancet last week showed how cannabis is more harmful than drugs such as LSD and ecstasy - but less so than alcohol and tobacco.

The number of people needing emergency treatment due to cannabis has virtually doubled in five years - from 581 in 2001 to near 1,000 last year.

But not everybody has welcomed the debate. Pro-legalisation campaigners claim the evidence for cannabis's damaging effects shows an association between the drug and psychosis, but not that one is the cause of the other. A more likely explanation, they argue, is that people in the early stages of mental illness may use the drug as a form of self-medication.

Roland Hyams a music PR and a supporter of decriminalisation, said: "I think alcohol is far more to blame for mental illness than cannabis. I've smoked plenty of skunk and never had problems."

However, some doctors now believe cannabis users have to take increasing quantities to experience the same "hit". A third of users are said to experience physical or mental withdrawal symptoms. One user in 10 is at risk of becoming an addict. "The strength of skunk means that users are more likely to become addicted to cannabis now," said Professor Peter Jones, professor of psychiatry at the University of Cambridge. "Withdrawal symptoms include anxiety, sleep disturbance and tremors."

J-Rock, from the rap group Big Brovaz and a former skunk addict, gave up the drug a year ago. He said: "Cannabis and skunk are definitely addictive and lead to psychological and physical changes in any person."

The 28-year-old musician started using when he was 13. When he came off the drug, he experienced headaches and mood swings. He describes the detox process as one of getting his sanity back. He says that skunk-induced paranoia is behind the surge in violent crime: "If you're on skunk and you have a confrontation with somebody, you feel almost untouchable."

Is this the evidence that shows cannabis is bad for your brain?

The controversial findings, published in the 'Journal of Psychoactive Drugs', give an indication of the possible effects of cannabis.

Images of the brain were created by single-photon-emission computerised tomography, which works by looking at blood supply and brain activity. Patients are injected with a radioactive substance which goes into the bloodstream and shows up on a brain scan. 'Mottled' areas represent areas where brain function is weakened . But some doctors question the validity of the images.

Normal brain

The brain image has a relatively smooth, uniform surface, with little indication of a loss of any brain function. It is the brain of someone who does not use cannabis and is an example of what an average brain can look like.

The 16-year-old who smokes every day

This scan, which shows a marked difference from the normal brain, is of a teenager who began using cannabis daily at 14 years of age. The image suggests that after just two years of using the drug, the brain is already affected

The 18-year-old who smoked for two years

The brain of a teenager who started using cannabis when 16 is badly pitted and scarred. The subject took the drug several times a week. Like the previous scan, this also apparently shows damage after two years of cannabis use.

The 28-year-old who smoked for 10 years

This scan shows how long-term damage can be caused by cannabis use. The subject had smoked skunk for 10 years. As with the other images shown, it appears to show decreased activity in the pre-frontal cortex and temporal-lobe areas.