Cannabis can be a hazy subject. While promising insight it can easily lead to confusion. Is it recreational or dangerous? Should it be policed like Ecstasy or nicotine? Does it ease pain or cause illness? In respect to our children, should we be worried or relaxed? Charles Clarke faces questions such as these as he decides whether, in the light of fresh medical evidence, to reverse the Government’s downgrading of cannabis two years ago to a Class C rather than a Class B drug.

David Blunkett, Mr Clarke’s predecessor as Home Secretary, was responding to decades of pressure when he softened the official view of cannabis. But science moves on apace. A few may believe that taking drugs is risk-free. They include the deluded, those who have spent too much time smoking the stuff, and, unfortunately, the young. Too many fall into all three categories. But there is now a growing body of research to suggest that the harmful effects of cannabis on mental health, particularly in teenagers, are only now being fully understood.

Medical findings have come thick and fast. Last year scientists in Denmark, New Zealand, Britain and the US suggested significant but qualified links between cannabis use and schizophrenia. The common thread is that those, especially teenagers, who are genetically or socially predisposed to mental illness could find that disease triggered earlier and with exacerbated symptoms if they are regular cannabis users. Some contend that cannabis can cause mental illness in any user, though the evidence is far more contentious. There are separate but linked concerns about potency. In its skunk variety, a joint smoked behind today’s bike sheds can contain up to 30 times more tetrahydrocannabinol, the cannabis chemical that stimulates the brain, than those puffed by Sixties hippies. It is the difference between sipping a pint and swigging a bottle of vodka.

When two thirds of 15-year-olds have been offered cannabis and every fourth one has tried it, prohibition has hardly succeeded. If the cannabis trade were controlled and licensed, people could make informed choices. Users would know what strength they were buying and that their vice of choice had not been cut with elephant tranquilliser. Drug barons would suffer. The unhealthy hypocrisy of outlawing something which the State cannot be bothered to police would be swept away at a stroke.

The logic is libertarian, but the leap is still a step too far. For now, alcohol remains our socially acceptable lubricant. And arguments about whether cannabis should be treated like Valium, a Class C drug, or amphetamines, Class B, largely miss the point (although the previously liberal Netherlands is considering stiffening the classification of skunk). Too many children are smoking cannabis at an age when their brains are not fully formed and when the harmful effects are most pronounced. One obvious step that Mr Clarke should take is to improve the weight and effectiveness of drug education in schools. The most vulnerable are in need of greater protection.