Defying the weight of medical and scientific opinion, Gordon Brown is to order tougher new laws today on cannabis possession. The Prime Minister has decided to overrule his own expert advisers and reverse the downgrading four years ago of Britain's favourite illegal drug from a class B to a class C substance, threatening cannabis smokers with five-year prison terms.

Headlines proclaiming that skunk is 20 to 30 times as strong as the cannabis smoked in the 1970s have fuelled public alarm. The Prime Minister has spoken of the "lethal" effects of the new strains, as if they were comparable with the harm caused by heroin and crack cocaine.

Yet there is no epidemic of psychosis – rates have actually declined since the mid 1990s. Almost three million people a year use cannabis; very few develop psychosis. Despite this, parents are more worried today that their children will become schizophrenic if they smoke cannabis than they were five years ago.