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

Related articles

• Biggest study yet links cannabis to psychosis
• Marijuana may be more damaging than thought
• How cannabis makes you forget
• Hothouse flowers
• Marijuana may slow progression of Alzheimer's

Does this prove marijuana causes schizophrenia? "The jury is still out," says Iversen. "The problem is that only a small proportion of people smoke heavily, and only a small proportion of people develop schizophrenia – we're dealing with vanishingly small numbers."

Given the large increase in the number of people who use marijuana over the past 30 years, as well as its increased potency, if marijuana was causing schizophrenia we should see an increased number of cases. But estimates by Hall and others show that the rate has stayed at about one per cent.

Many researchers believe the most plausible explanation of this data is that marijuana does not trigger schizophrenia in those who would otherwise not develop it. Rather, marijuana use brings on symptoms earlier and more severely in patients who would get the disease anyway. Says Castle, "It seems to be the straw that breaks the camel's back."

To find out who these susceptible people are, a New Zealand study looked to their genes. The study showed that people who carried a particular variety of a gene called catechol-o-methyl transferase (a gene involved in the synthesis of the brain chemical dopamine), were about 11 times more likely to develop schizophrenia if they used marijuana.

But in most cases of schizophrenia, researchers can't pinpoint the genes responsible. The best guide as to who is most at risk from smoking marijuana is a family history of mental illness or existing psychotic symptoms.

READ OUR ARTICLE ON MEDICAL MARIJUANA HERE.

MARIJUANA USE has also been linked to depression. A 2002 study published in the British Medical Journal by George Patton, of the Centre for Adolescent Health at the University of Melbourne, showed that marijuana use doubled the risk of later depression and anxiety in teenage girls.

Other studies, like a 2005 paper in the journal Addictive Behaviours by Thomas Denson at the University of Southern California and Mitch Earleywine of the University of Albany found the opposite: it was entitled "Decreased depression in marijuana users." Wayne Hall agrees that the link between marijuana and depression is even more open to question than that between marijuana and psychosis.

See our boxout that goes with this feature – Medical Marijuana, read the full article here.

As for addiction, the common wisdom is that marijuana is not an addictive drug. The laboratory rat will attest to that: put heroin, cocaine or amphetamines into its water bottle and the rat will keep coming back. Put the main active component of marijuana, THC (delta-9-tetrahydrocannabinol) in its food and it won't.

Yet, scan through the scientific journals and the academic literature and you'll find a surprising term: 'marijuana dependence'.

"Whether or not there is a cannabis dependence syndrome is one of the most contested claims in the cannabis policy debate," admits Hall.

Adds Paul Dillon, a spokesman for Australia's National Drug and Alcohol Research Centre (NDARC): "If you talk about a drug of dependence, that heightens attention and the chance that the legal ramifications will be upped. A lot of people simply don't consider marijuana to be that harmful."

According to Earleywine, author of the books Understanding Marijuana, and the soon-to-be-published Pot Politics, the changes in terminology – which evolved from 'physical addiction' to 'psychological dependence' to 'dependence' – might have been an attempt to get marijuana labelled as a true drug of addiction.

"The bottom line is that marijuana never quite qualified as such, so they started making distinctions. Nobody ever liked the 'psychological dependence' idea, as it was meaningless and [people] started applying it to running and love and everything else. Rumour has it that the whole thing was all about getting marijuana users diagnosed as dependent."

But addiction is characterised not just by a craving for more or increased physiological tolerance to exposure, but also by withdrawal symptoms. "People were still trying to find marijuana withdrawal after 4,000 years of use," notes Earleywine.

On the other hand, Hall says that the changing perception of marijuana from a non-addictive drug to a drug of dependence was just a natural consequence of the passage of time. "This is what we saw when cocaine and amphetamines started being used. They weren't considered addictive drugs. Cannabis is just repeating history."

For a snapshot of what marijuana dependence actually looks like, take a visit to one of the burgeoning clinics in Britain, the United States, Australia and Amsterdam. According to a 2001 study by the Australian Institute of Health and Welfare, the fraction of patients who attended drug clinics for marijuana dependence rose sharply, from
4 per cent to some 21 per cent over 10 years.

"There's no question that some people get into strife and can't stop," says Hall. "A dependent marijuana smoker is like a heavy drinker or smoker: they are chronically intoxicated and spend as much as 25 per cent of their income on it. Often we look at people in their early 30s, who may have smoked daily for 15 years without knowing there was a problem. Then they try to stop, and they can't. Other typical cases are young boys in juvenile detention or in the mental health service. They smoke
20 to 30 bongs a day, every day."

Controversial or not, the terminology change does not seem to have been part of a conspiracy to demonise marijuana. In the 1970s, the World Health Organisation pushed for a terminology change to remove the stigma attached to the word 'addict'. And as new drugs came out of fields and laboratories and into widespread use, the term 'addiction' became outdated because it was specifically tailored to the use of heroin and opium (known jointly as opiates). When amphetamines or cocaine came along in the late 1970s, they didn't fit this description, and so were not considered harmful drugs. Eventually, says Hall, their harmful effects became apparent. And better checklists were required.

Over the past 20 years or so, several checklists have come and gone. The latest and most widely used is the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders, currently in its fourth version and generally abbreviated as DSM IV. This checklist looks at the broad impact of the drug on the person's life as well as the psychological and physical fallout. For instance, does it disrupt their professional or personal life? Do people use it despite knowing it causes them problems? Do they spend a great amount of time and money trying to get the drug? Do they find it difficult to give up? There are seven items on the checklist; if the user scores three of them in the previous 12 months, they are considered dependent.

On this radar screen, marijuana certainly makes an appearance. According to Hall, about one in 10 people who sample marijuana will become dependent on it. But let's keep things in perspective: for cigarettes the comparable figure is one in three, for alcohol one in six, and for heroin it's one in four.

Looked at this way, marijuana is but a minor offender. However, given the large number of marijuana users, the figures start to climb. According to a 2001 study by Wendy Swift at NDARC, an estimated 300,000 Australians are marijuana dependent. Certainly the briefing information for the Australian National Cannabis Strategy, an attempt to come up with a plan for dealing with cannabis that delivered its report in May 2006, describes cannabis as "second only to heroin use in terms of healthy years of life lost ... there were more disability-adjusted healthy years of life lost in 1996 due to cannabis use and dependence than [due to] HIV, hepatitis B and hepatitis C."

Still, according to Earleywine, "that alarming equation smacks of statistical hand waving; a few assumptions turned into an equation that sounds more alarming than it really is. Marijuana withdrawal symptoms are things like irritability and not having the munchies [food cravings]. Of all the things I'd rather have, cannabis dependence is way above those others." As to the evidence that cannabis causes significant losses of time in employment or in relationships (the so-called 'amotivational syndrome'), Earleywine contests the data. He cites his own study on a group representing the general population – Internet users – that was published in January 2006 in the journal Substance Abuse Treatment, Prevention, and Policy. It showed that regular marijuana users didn't necessarily take more sick days or earn less money.

The Mardigrass Festival campaigns for the legislation of marijuana as well as offering five days of cannabis-based activities in the New South Wales town of Nimbin.(Image: Nimbin Hemp Embassy)

AS TIME HAS passed, cannabis dependence syndrome has been described not only in humans, but in rats too. In the past, rats were reassuring because they didn't get addicted to THC when it was added to their food. But according to the traditional rat test, nicotine isn't addictive either – and anyone who has tried to give up smoking tobacco will certainly disagree with that. So could it be that there's something wrong with the test?

Just as people are now given a different test to measure dependence, so are rats. If rats are put in a cage with a red and a green room, and they are injected with THC only in the green room, they will end up spending more time in the green room. Arguably, it mimics the behaviour of a dependent person: they change their life activities to acquire the substance. On this test, rats are revealed as forming dependence on both nicotine and cannabis.

Readers' comments