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Cannabis and the Brain

Across Europe and America, there is a groundswell of debate concerning the legalization of cannibas for personal use. Indeed, Britain has, to all intents and purposes, practically decriminalized marijuana usage. As a neuro-scientist, I am concerned about this debate.

One common justification for legalization or decriminalization of cannabis centers around the idea that it does not involve a victim. But at least four reports in major medical journals (Ramstrom, 1998; Moskowitz, 1985; Chesher, 1995; and Ashton, 2001), show the contrary. In a study of pilots smoking only a single moderate joint, there was a difference between a placebo control group and those taking cannabis, up to 50 hours after taking the drug. Other costs to the community are accidents at work or at home, educational under-attainment, impaired work performance and health budget costs.

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Another argument for relaxing our attitude to cannabis is that it is non-addictive. Of course, defining `addiction' is hard, but if one regards it as an inability to give up, then there is strong evidence that cannabis incites dependence. Recent papers report many users in the US, UK and New Zealand now seek treatment for dependence. Other papers show that 10% of users want to stop or cut down, but have difficulties doing so, whilst a paper in 1998 reported that 10-15% of users become dependants.

Recently, it was shown that withdrawal symptoms were experienced after only three days of light use. Heavy users confront a worse situation: Dr Bryan Wells, a rehabilitation expert, comments, ``for the first time I'm beginning to see something that resembles the withdrawal symptoms produced by hard drugs in heavy cannabis users.''

Another argument in favor of relaxed laws on cannabis are its supposed beneficial effects on pain. So far, that evidence is anecdotal; it is hard to exclude placebo effects. The results from clinical trials are awaited. But distinctions should be drawn between recreational drugs and medicines, as they are for opiates. If cannabis is a pain-killer, then it must have a huge impact on the physical brain.

Indeed, widespread reports exist of the impact of cannabis on the brain, in particular areas concerned with memory (hippocampus), emotion (mesolimbic system), and movement (basal ganglia). Cannabis affects a variety of chemical systems and it works via its own `receptor,' it's own molecular target. The fact that there is a naturally occurring analogue of cannabis in the body, as there is for morphine, provides a basic reason to differentiate it from alcohol.

For an agent that affects a variety of transmitter systems, is as though it were a transmitter itself. Perhaps, not surprisingly, for cannabis has a clear effect on psychology: not only is there euphoria, but often overlooked effects of anxiety, panic and paranoia. Disorders in psychological performance, attention impairments, and memory deficits are well known.

More disturbing - and less acknowledged - is the fact that these effects can be long-term. In one recent paper, a comparison of ex-users who used for 9 years and abstained from 3 months to 6 years, were compared with long-term users of 10 years, and short-term users of 3 years. In all cases the frequency of use was 10-19 days per month. In all cases, all users, irrespective of whether they had given up or not, had attention impairments compared to controls.

Although those who stopped using cannabis partially improved over those continued using cannabis, they were below the level of controls, and this impairment was related to the duration of use. Most disturbing was the fact that no improvement in performance occurred with increasing abstinence.

No surprise, then, that because these long-term effects seem to be irreversible, there is an effect on brain pathology. Because much of this data comes from work with isolated systems, and therefore on all brains, an obvious criticism is that you can't extrapolate from such data. Yet, the evidence suggests that the long-term effects must have a physical basis.

A counter-argument to such thinking is to challenge whoever thinks that there is a `safe' dose of cannabis, with no effect on the brain, to say what that dose might be. Even a dose comparable to one joint, and analogous levels of the active THC ingredient to that in plasma, can kill 50% of neurons in the hippocampus (an area related to memory) within 6 days.

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People, moreover, are unaware that the THC in cannabis remains in the body for over 5 days. For someone using cannabis routinely, the dose carried in the body is higher than imagined. It is also easy to underestimate the dose being taken, because there is a wide variety in the strength of cannabis. Individual variations in body fat and, worryingly, disposition to psychosis, means that you cannot predict how much will affect any person at any time.

Cannabis could well be having a serious effect on the mind, which I define as the personalization of brain circuits that reflect an individual's experiences. A transmitter-like substance, with such powerful effects, must affect those circuits. So `blowing your mind' might be exactly what marijuana users do.