Why are we asking this now?

Yesterday, the Advisory Council on the Misuse of Drugs began reviewing scientific evidence on the classification of cannabis, amid widespread fears that Britain is in the grip of an epidemic of cannabis-induced psychosis. This view is based on the belief that cannabis sold on the streets is stronger than it was a generation ago and is tipping vulnerable people into mental illness, including schizophrenia.

The review, the second in two years, was ordered by Gordon Brown, who has indicated he is minded to reverse the decision of the former Home Secretary David Blunkett, who downgraded the drug from Class B to Class C in 2004. Mr Blunkett's aim was to free up police time squandered on prosecuting users, but the Prime Minister is concerned that the move sent the wrong message to young people, who are now confused about drug laws.

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What is skunk?

It is the generic name given to potent strains of the cannabis plant containing the highest levels of the psychoactive ingredient tetrahydrocannabinol (THC). The original skunk, a cross between the fast-growing Indica and the potent Sativa strains, is believed to have originated in the US and was so called because of its pungent smell. Dutch growers have since refined and cross-bred the plants to produce a number of strains including super-skunk, Early Girl, Northern Lights and Jack Herer. They are normally grown indoors under lights, or in a greenhouse. The more intensive the cultivation, the higher the THC content.

How strong is skunk?

Traditional herbal cannabis contains 2 to 4 per cent THC, according to the Drugscope charity. More potent varieties average 10 to 14 per cent – three to four times as strong. Claims that skunk is 20 to 30 times as powerful as herbal cannabis are exaggerated. A European review of cannabis potency in June 2004 concluded that the overall potency of cannabis products on the market had not increased significantly because imported cannabis dominated the market in most countries. However, as home-grown cannabis has become more widely available, especially in the Netherlands and Britain, consumption of stronger varieties has increased. The Home Office yesterday claimed that 70 per cent of cannabis sold on the streets of Britain was skunk.

Why is skunk so popular?

It offers a powerful high, similar for some users to that obtained with ecstasy or LSD but without the chemicals. Even in the world of recreational drugs, users prefer a "natural" high. It can be home-grown from legally obtainable seeds – many users grow a few plants on a windowsill for personal consumption. It is easy to identify, making it difficult to fake. Cannabis resin, by contrast, has been adulterated with everything from boot polish to the horse tranquilliser ketamine.

How dangerous is skunk?

This is an area of intense dispute. The greatest concern is over its effects on mental health. When the advisory council last reviewed cannabis in early 2006, it concluded that use of the drug by existing schizophrenia sufferers might worsen their symptoms and lead to a relapse in some. But on causation, it said: "The evidence suggests, at worst, that using cannabis increases the lifetime risk of developing schizophrenia by 1 per cent."

Some scientists believe this underplays the risks. Professor Robin Murray, of the Institute of Psychiatry in London, says that in vulnerable individuals – those with an unsuspected genetic predisposition to schizophrenia – cannabis may be the trigger that starts a full-blown mental illness.

His research showed that up to 10 per cent of the adult population – or about four million people – are prone to paranoid thoughts or grandiose ideas and, among those who smoke cannabis regularly, half may be tipped into psychotic delusions and end up needing treatment. The early age at which people start smoking the drug today, compared with two or three decades ago, is an additional concern, he says, because their brains may be more vulnerable.

Have the risks been overstated?

Yes. A recent report, printed in several newspapers last month, suggested that 500 people a week were being admitted to hospital for treatment for the effects of cannabis – a 50 per cent increase since the drug was downgraded to Class C in 2004. The figure was repeated in reports this week. Drugscope said the figure, quoted by the Public Health minister Dawn Primarolo, was actually related to the number of people consulting community drug treatment services for help or advice where "treatment" may amount to no more than an informal chat. The actual figure for hospital admissions was 14 per week in 2006-07 – and that was lower than the year before.

So why did the advisory council recommend that cannabis remains a Class C drug?

Because, despite the evidence of a link with schizophrenia, it concluded that cannabis remained "substantially" less harmful than the Class B drugs amphetamines and barbiturates. The classification of drugs is, in part, about proportionality. Experts have long argued that the current system for ranking drugs – Class A for the most dangerous to Class C for the least dangerous – is irrational. In terms of harm caused, alcohol and tobacco are more dangerous than cannabis, yet they are legal and cannabis is illegal. Defenders of the 2004 decision to downgrade cannabis to Class C say that it has not increased its use and it has freed police to tackle drug-dealers and other more serious crimes. Since 2004, consumption of cannabis has actually declined.

Are there other risks from skunk?

Yes – but they are small. Skunk is smoked like cannabis, contains carcinogenic substances and is often mixed with tobacco. The British Lung Foundation estimated that smoking three joints was equivalent to smoking 20 cigarettes but most users give up after a few years. Some experts also believe there is a risk of dependence among regular smokers. However, cannabis is not seen as a drug of addiction like heroin or tobacco.

The greatest fear, alongside its effect on mental health, is that skunk may lead people on to use harder drugs. It is certainly true that many people who become heroin addicts have used cannabis in the past. But the vast majority of cannabis users never progress to heroin. Research by the Home Office concluded that the so-called "gateway effect" of cannabis – leading users on to harder drugs – was probably "very small".

Should the newer, more potent forms of cannabis be re-classified?

Yes...

* Vulnerable individuals with a predisposition to mental illness may be tipped into psychosis by the drug

* The younger age at which people start smoking means their brains are more vulnerable

* Skunk cannabis is three to four times stronger than herbal cannabis and has more potent effects

No...

* At worst, the experts say, the use of cannabis increases the lifetime risk of schizophrenia by only 1 per cent

* The chances that users of skunk will progress to harder drugs such as heroin are very small

* Skunk is safer than alcohol and tobacco, which are legal and cause more than 100,000 deaths a year