A wide variety of self-appointed experts, who have no idea of what really goes on in hospitals, have constantly preached that cannabis is a safe drug - or, at least, that it is less dangerous than alcohol. Certainly the Government's demotion of cannabis from a class B to a class C drug last year sends the signal that it is less risky than we thought, that we over-reacted when we classed it as addictive or dangerous.
In fact, few clinicians in Britain today would argue that cannabis is safe.
It is an extremely dangerous drug that can induce severe psychotic illness in which the user hears voices and develops delusions. People who take the drug can also become extremely unpredictable.
The dangers of cannabis use are exemplified by a recent investigation that showed cannabis abusers were eight times more likely than the general population to make a serious suicide attempt. Research also suggests that at least one in seven cannabis users reports strange and unpleasant experiences, such as hearing voices or developing a conviction that someone is trying to harm them. Regular cannabis use also trebles your chances of developing a psychotic illness and being diagnosed with schizophrenia.
If you have tried cannabis relatively frequently, just 50 times in your life, your chances of developing this disorder are increased sixfold. Many make a comparison with alcohol and argue that cannabis is not as dangerous.
But while alcohol is by no means itself a safe substance, it does not have the propensity to cause such serious mental illness after such relatively low levels of use. Many people also argue that, unlike heroin or cocaine, cannabis is not addictive. But about 10 per cent of regular users are clearly addicted to it and suffer severe withdrawal if they try to stop.
Indeed, cannabis dependence was recently shown to be the most prevalent illicit substance misuse disorder among North American and Australian adults.
The chattering classes tend to be wary of sounding tough on cannabis as they know their children are likely to have tried it. Cannabis use among university students appears to have at least doubled between the 1980s and 1990s; a clampdown would criminalise a large proportion of their friends and family.
Drug liberals point to the Netherlands as a shining example of relaxed drug laws. But since 1995, the Dutch government has been trying to reverse its drug liberalisation policy. Recent changes include dramatically reducing the maximum amount sold legally in a coffee shop to 5 grams, down from 30 grams, and increasing the legal age limit for use from 16 to 18. But teenagers say it's now easier to buy the drug than it was in 1996.
Furthermore, three recent studies revealed that difficulties with age checks mean under-age adolescents can still get the drug from coffee shops.
Given the ease with which under-16s can get hold of cigarettes in Britain, one of our primary concerns should be the impact on our children of decriminalising cannabis. The evidence suggests that, even if you don't become psychotic or develop other mental illnesses linked with cannabis, its use causes intellectual and motivational impairments, particularly in young people. For example, one recent study found that pupils who smoked cannabis were seven times more likely than the rest of their peers to leave school, and six times more likely to have played truant.
Given these devastating effects on children, it is vital that adults shield them from the dangers. We can start by admitting the facts: cannabis is not safe and no Government policy should ever send that message.
Professor Raj Persaud is a consultant psychiatrist at the Bethlem Royal and Maudsley hospitals in London. His latest book is The Motivated Mind (Bantam Press, pound;12.99). Email: firstname.lastname@example.org