Debunked: politicians' excuse that cannabis has become stronger

Debunked: politicians' excuse that cannabis has become stronger

By Jeremy Laurance, Health Editor
Published: 21 July 2007

In a week in which Gordon Brown signalled a toughening of the law on cannabis and Labour MPs queued up to confess to smoking dope in their youth - a dozen cabinet ministers at the last count - there has been a widespread assumption bandied about that the country is in the grip of an epidemic of cannabis-induced psychosis.

But there is no evidence that cannabis poses a greater threat to health today than it did 30 years ago, and reports that stronger forms of the drug, called skunk, have 25 times the potency are wildly exaggerated. The joint, symbol of peace and love in the 1960s, has become a totem of degenerate Britain - increasingly linked with mental breakdown and axe-wielding maniacs.

The Prime Minister, who has ordered the second review of the classification of cannabis in two years, is said by insiders to want to reverse the decision of the former home secretary, David Blunkett, who downgraded the drug from class B to class C in 2004.

The Advisory Council on the Misuse of Drugs, which examined the issue 18 months ago, will be asked to do so again. It concluded in its report in December 2005 that the strength of cannabis resin (hash) had changed little over 30 years and was about 5 per cent tetrahydrocannabinol (THC). Skunk, it found was 10 to 15 per cent THC - two to three times as strong, not 25 times.

Professor Leslie Iversen, a pharmacologist at Oxford University, said the widespread belief that skunk was 20 to 30 times as powerful was "simply not true".

The biggest change over recent decades has been in the strength of indoor-cultivated herbal cannabis, but even this has only doubled to 12 to 14 per cent THC. Although exceptionally strong skunk can be found on the market in Britain, it always has been available, according to reports from the UN Drug Control Programme.

On the question of psychosis, the advisory council was clear. Cannabis use may worsen the symptoms of schizophrenia and lead to a relapse in some patients. But on causation, it said: "The evidence suggests, at worst, that using cannabis increases the lifetime risk of developing schizophrenia by 1 per cent."

It added that more than three million people were estimated to have used cannabis in the previous year, but "very few will ever develop this distressing and disabling condition".

Scientists led by Professor Murray, at the Institute of Psychiatry, have argued that cannabis smoking can trigger psychosis in vulnerable individuals. A key worry is that young people are starting to smoke the drug earlier, in their mid-teens, when their brains are more vulnerable.

But experts led by Professor David Nutt, a specialist in addiction psychiatry at the University of Bristol, said in The Lancet in March that a causal link had not been established. Even if it were, cannabis could account for at most 7 per cent of cases of schizophrenia, he said.

One difficulty is distinguishing an association from a cause. Marjorie Wallace, the chief executive of Sane, the mental health charity, was quoted as saying evidence of the damaging effects of cannabis was mounting, with psychiatrists claiming "80 per cent of patients have been using cannabis". Cannabis use is widespread among mental patients, but their attempt to self-medicate tends to be a symptom of their suffering, not the cause.