Cannabis literature from Holland, where it's legal to buy small quantities of the drug for personal use.

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It's a surprise to find a poster of brightly coloured marijuana leaves adorning the office of Melbourne psychiatrist David Castle. After all, Castle – a professor at St Vincent's Hospital – is, with colleague Robyn Murray of London's Institute of Psychiatry, the author of the prize-winning tome, Marijuana and Madness, a collection of scientific essays on the link between marijuana and schizophrenia.

As we sit in his bright, roomy office, the youthful, denim-clad psychiatrist talks in his clipped South African accent, effusing sympathy for the plant that causes so much heartache to parents and endless social debate.

The sympathy is inherited, it seems. His mother, a renowned Cape Town doctor who was noted for her "interest in things slightly off the edge", explored the medicinal uses of the narcotic weed during the 1950s. Castle has continued the family tradition.

As our discussion weaves through the data, I struggle at first to divine Castle's message. Finally, it dawns on me. The psychiatrist is … I think … exasperated. "When it comes to the marijuana debate, science and rationality have very little to do with it: the truth about marijuana has been lost in the smoke of political rhetoric," Castle tells me.

He is not alone. The 'Marijuana Wars' have waged for decades and there are numerous veterans who also have that exasperated sound.

WAYNE HALL IS former director of Australia's National Drug and Alcohol Research Centre (NDARC), and now a professor at the school of population health at the University of Queensland. Australia has probably done more research on marijuana than anywhere else: a result of its high rate of use and ample research funding, Hall tells me. He has pondered the harms of marijuana for the last 13 years and his commentaries appear in prestigious journals, like a recent one on cannabis and schizophrenia in the January 2006 issue of The Lancet.

"It's hard to get the real message out because the debate is so polarised. If it is perceived to be harmful, people want to go to war and lock up every user; if it is perceived to be harmless, they want to legalise it completely. The truth is that cannabis is a drug like any other – some people will experience difficulty," says Hall.

It seems that after all the textbooks, the scientific papers, and the front-page headlines, it's still the same old story: marijuana used in moderation is a relatively harmless drug. Pharmacologist Les Iversen, now a visiting Oxford scholar, tells me, "Marijuana is somewhat more harmful than aspirin." Iversen should know; he spent 10 years assessing the risks of drugs for pharmaceutical giant Merck & Company, and recently served on Britain's Advisory Council on the Misuse of Drugs.

On the scale of harmful substances, marijuana ranks fairly low. Tobacco and alcohol exact a far greater toll, between them accounting for some 12 per cent of global deaths. Even aspirin is credited with causing in the vicinity of 50 deaths a year in Australia alone. No deaths are attributable to marijuana.

Yet in the war on drugs, marijuana continues to be singled out as the principal scapegoat. In 2005, some three quarters of a million people in the U.S. alone were arrested on marijuana-related charges, about 89 per cent of these just for possession. Even cancer patients who were using marijuana to ease their symptoms were among those arrested.

In Australia, the state of New South Wales recently toughened its laws, and Prime Minister John Howard has called for more to follow: "I will ask [the state premiers] to agree with me that part of the solution to the mental health problem is a tougher line on marijuana, and I imagine they will agree with me," he said, ahead of a forthcoming summit with the states.

Marijuana has been demonised, says Castle, to the point that even its considerable medicinal and agricultural uses have been disavowed. The plant is extremely hardy, and has as much to offer the environmentally challenged world today as it did in times of old, when 'Indian hemp' provided the mainstay for ship sails and rope.

The cover of Life magazine, October 1969.

SOME PUT the scapegoating of marijuana down to the conservative political tide sweeping Western democracies. Until 2004, Peter Cohen was the director of the Centre for Drug Research at the University of Amsterdam. The Netherlands, as any backpacker will tell you, is famous as one of the few places in the world where you can legally buy cannabis. But marijuana use is still technically illegal: individuals can only buy up to 5 grams, and advertising its sale is not permitted.

For 20 years, Cohen's centre documented the country's experiment of decriminalising cannabis, allowing small quantities to be sold from the coffee shops that now outnumber butcher shops in parts of Amsterdam. His studies show drug laws have very little impact on the rate of cannabis use anywhere in the world. "There are enormous differences in the rates of drug use between countries," he says. "We don't really know why. For instance in the Netherlands, 17 per cent of people have tried it; the U.K. rate is twice as high. We don't think drug policy has anything to do with it."

In his view, the direction of marijuana research is at the whim of the political climate. "There's a fashion now to determine the cause of mental health problems; they want research that shows the harms of marijuana. But people's lives don't fall apart because of marijuana. I call it Soviet science – science geared to produce a political point."

Alex Wodak is director of the Alcohol and Drug Service at St. Vincent's Hospital in Sydney and Australia's most colourful drug-war resistance hero. He agrees politics is a big driver in the marijuana wars. "[Australian Prime Minister John] Howard can't lose on this. If he wins, he'll be wrapped in the Australian flag as protecting the youth of the future. If he loses, then the states [who are his political enemies] get labelled 'soft on drugs'."

Yet even if most people are not harmed by smoking marijuana – and, as Iversen puts it, "many find it a very useful drug" – there's no doubt a minority is harmed.

And that minority is recognisable to many. It might be the vulnerable teenager for whom marijuana is the grease that slides them towards aimless drug-dependence; psychiatrists will think of their mentally ill patients whose marijuana habit makes their disease much worse; and then there is that minority in the general population, largely undetectable, who have a predisposition to schizophrenia. Most psychiatrists now believe marijuana smoking will push those susceptible to schizophrenia over the edge.

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