What is really needed in dealing with cannabis is a “tobacco moment”, as with cigarettes 50 years ago, when a majority of people became convinced that smoking might give them cancer and kill them. Since then the number of cigarette smokers in Britain has fallen by two-thirds.

A depressing aspect of the present debate about cannabis is that so many proponents of legalisation or decriminalisation have clearly not taken on board that the causal link between cannabis and psychosis has been scientifically proven over the past ten years, just as the connection between cancer and cigarettes was proved in the late 1940s and 1950s.

The proofs have emerged in a series of scientific studies that reach the same grim conclusion: taking cannabis significantly increases the risk of schizophrenia. One study in The Lancet Psychiatry concludes that “the risk of individuals having a psychotic disorder showed a roughly three times increase in users of skunk-like cannabis, compared with those who never used cannabis”.

As 94 per cent of cannabis seized by the police today is super-strength skunk, compared to 51 per cent in 2005, almost all those who take the drug today will be vulnerable to this three-fold increase in the likelihood that they will develop psychosis.

Mental health professionals have long had no doubts about the danger. Five years ago, I asked Sir Robin Murray, professor of psychiatric research at the Institute of Psychiatry in London, about them. He said that studies showed that “if the risk of schizophrenia for the general population is about one per cent, the evidence is that, if you take ordinary cannabis, it is two per cent; if you smoke regularly you might push it up to four per cent; and if you smoke ‘skunk’ every day you push it up to eight per cent”.

Anybody wondering what happens to this 8 per cent of the skunk-smoking population should visit any mental hospital in Britain or speak to somebody who has done so. Dr Humphrey Needham-Bennett, medical director and consultant psychiatrist of Cygnet Hospital, Godden Green in Sevenoaks, explained to me that among his patients “cannabis use is so common that I assume that people use or used it. It’s quite surprising when people say ‘no, I don’t use drugs’.”

The connection between schizophrenia and cannabis was long suspected by specialists but it retained its reputation as a relatively benign drug, its image softened by the afterglow of its association with cultural and sexual liberation in the 1960s and 1970s.

This ill-deserved reputation was so widespread that even 20 years ago, the possible toxic side effects of cannabis were barely considered. Zerrin Atakan, formerly head of the National Psychosis Unit at the Maudsley Psychiatric Hospital and later a researcher at the Institute of Psychiatry, said: “I got interested in cannabis because I was working in the 1980s in an intensive care unit where my patients would be fine after we got them well. We would give them leave and they would celebrate their new found freedom with a joint and come back psychotic a few hours later.”

She did not find it easy to pursue her professional interest in the drug. She recalls: “I was astonished to discover that cannabis, which is the most widely used illicit substance, was hardly researched in the 1990s and there was no research on how it affected the brain.” She and fellow researchers made eight different applications for research grants and had them all turned down, so they were reduced to taking the almost unheard of course of pursuing their research without the support of a grant.

Studies by Dr Atakan and other psychiatrists all showed the connection between cannabis and schizophrenia, yet this is only slowly becoming conventional wisdom. Perhaps this should not be too surprising because in 1960, long after the link between cigarettes and lung cancer had been scientifically established, only a third of US doctors were persuaded that this was the case.

A difficulty is that people are frightened of mental illness and ignorant of its causes in a way that is no longer true of physical illnesses, such as cancer or even HIV. I have always found that three quarters of those I speak to at random about mental health know nothing about psychosis and its causes, and the other quarter know all too much about it because they have a relative or friend who has been affected.

Even those who do have experience of schizophrenia do not talk about it very much because they are frightened of a loved one being stigmatised. They may also be wary of mentioning the role of cannabis because they fear that somebody they love will be dismissed as a junkie who has brought their fate upon themselves.

This fear of being stigmatised affects institutions as well as individuals. Schools and universities are often happy to have a policy about everything from sex to climate change, but steer away from informing their students about the dangers of drugs. A social scientist specialising in drugs policy explained to me that the reason for this is because “they’re frightened that, if they do, everybody will think they have a drugs problem which, of course, they all do”.

The current debate about cannabis – sparked by the confiscation of the cannabis oil needed by Billy Caldwell to treat his epilepsy and by William Hague’s call for the legalisation of the drug – is missing the main point. It is all about the merits and failings of different degrees of prohibition of cannabis when it is obvious that legal restrictions alone will not stop the 2.1 million people who take cannabis from going on doing so. But the legalisation of cannabis legitimises it and sends a message that the government views it as relatively harmless. The very fact of illegality is a powerful disincentive for many potential consumers, regardless of the chances of being punished.

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The legalisation of cannabis might take its production and sale out of the hands of criminal gangs, but it would put it into the hands of commercial companies who would want to make a profit, advertise their product and increase the number of their customers. Commercialisation of cannabis has as many dangers as criminalisation.

A new legal market in cannabis might be regulated and the toxicity of super-strength skunk reduced. But the argument of those who want to legalise cannabis is that the authorities are unable to enforce regulations when the drug is illegal, so why should they be more successful in regulating it when its production and sale is no longer against the law?

The problem with these rancorous but sterile arguments for and against legalisation and decriminalisation is that they divert attention from what should and can be done: a sustained campaign to persuade people of all ages that cannabis can send them insane. To a degree people are learning this already from bitter experience. As Professor Murray told me five years ago, the average 19- to 23-year-old probably knows more about the dangers of cannabis than the average doctor “because they have a friend who has gone paranoid. People know a lot more about bad trips than they used to.”

Patrick Cockburn is the co-author of Henry’s Demons: Living With Schizophrenia, A Father and Son’s Story