Concerns about cannabis should be taken seriously, but they do not justify lurching back to the failed policies of the past.
Tom Clark

David Blunkett is not remembered for having been a liberal home secretary. It was evidence and argument that persuaded him, at the start of 2004, to downgrade cannabis from being a class B to a class C banned drug. It is, however, politics that lies behind Gordon Brown's suggestion today that the move may be reversed. The change would be a significant one. Possession of cannabis is no longer, in general, an arrestable offence. If it were restored to class B, however, a prison sentence of up to five years for mere possession could apply (though sentences would rarely be that stiff in practice).

The biggest driver of the 2004 change was that the old approach had failed. Before the war on drugs was declared in the early 1970s, cannabis was rarely found outside universities. For instance, in 1967 - the first summer of love - there were in fact just 2,293 convictions. Despite the threat of stiff penalties, however, it soon grew to become mainstream. By 2004 there were 3.6m users, and another driver for the change was the hugely adverse effects - in terms of confidence in the justice system - of criminalising such a high proportion of the population. Then, finally, there were practical matters - the waste of police and court time involved in dealing with so many otherwise law-abiding citizens. Before the change cannabis alone accounted for 75% of police inquiries related to drugs.

Two developments underlie renewed anxiety about the drug. Both should be taken seriously, but neither justifies lurching back to the failed policies of the past.

First, a bunch of studies have now suggested that there is a real link between cannabis and mental illness for a minority of users, and especially for those who start using it young. These dangers should not be dismissed, but clearly highlighted - as should the lung damage that can follow from inhaling tar-filled smoke. But a Lancet study published as recently as March this year looked at the risks in the round and concluded that - while serious - the drug remained less of a problem than alcohol and tobacco. Yet no one disputes that the right way to focus the minds of drinkers on smokers on the risks they are taking is through information campaigns - not sending publicans or tobacconists to jail.

Second, an increasing proportion of the cannabis sold in Britain is skunk, a variety of the plant containing more of the active chemical (known as THC). Just as it is easier to get dangerously drunk on whisky, as opposed to beer, losing the plot on skunk is also easier to do. Some users will have the sense to recognise this, and put less of the stuff in each joint - just as sensible whisky drinkers do not pour pint-size measures. But there is no disputing that some skunk smokers will take things to excess. The idea, however, that the remote possibility of stiffer sentences will put off these people - the very users who are most foolhardy when it comes to risk - seems fanciful to me.

In 2005, Blunkett's successor, Charles Clarke, agreed to review the downgrading of the drug - seizing on the mental health issue as a way of trying to buy Labour some cover against the charge that it had gone soft on drugs, during that year's election campaign. But he reported back - and only last year - that class C status was appropriate. The more liberal approach had not increased use, he said. Indeed, the drug seemed to be less fashionable than before - whereas 28% of 16-24 year olds had smoked it in 1998, he told the Commons that in 2004/5, the first full year of the less punitive approach, that figure was down to just below 24%.

The government is today stressing that minds are not made up, and that it will look at the evidence again - and the involvement of the government's advisory council means that this should happen. But it is hard to see what has changed since 2006 - except the political weather. Following on from his move to ditch supercasinos last week, the son of the manse may be trying to signal that his government will give out strong signals - he has, after all, stressed that he is guided by a moral compass.

There are crucial differences, though, between the two cases. The casino move was courageous and welcome. It is not about driving gambling underground, but facing down the demands of big businesses that they should be given licence to promote it in new ways. With cannabis, however, the issue is how far to make criminals of the millions of individuals using it. The risks are real but getting tough will fail to address them - just as it has failed in the past. A moral compass pointing to a crackdown is one whose guidance should be ignored