Observations on cannabis

When cannabis was demoted last year from a Class B to a Class C drug, it seemed to me that the decision was influenced by a government closer to the metropolitan chattering classes than to what goes on among the socially marginalised people out in the sticks. In my work I see the worst problems of multi-drug users, and I find that one drug above all others is prone to precipitate acute mental derangement: cannabis. I know crack and heroin users who recoil in horror from smoking joints.

So I was not surprised to learn that Luke Mitchell, the 16-year-old convicted of the murder of his girlfriend Jodi Jones, had been bingeing on cannabis. He has been called evil, but his description has many of the hallmarks of psychosis: self-neglect, bizarre behaviour and complete lack of appropriate emotional response. And this is a boy who, up to the age of ten, seems to have been unremarkably normal.

One in five 15-year-olds in the UK smokes the weed. Though there is a dearth of research on its effects on the adolescent brain, evidence is accumulating that starting the habit early dramatically increases the risk of schizophrenia in the ensuing decade, and that the risk grows with frequency of use and strength of dose. An 18-year-old who has used cannabis 50 or more times suffers an elevenfold increased risk of schizophrenia in the next 15 years. For those who begin at 12 or below, with gargantuan doses, the risks are unquantifiable. In many US hospitals cannabis is now the commonest cause of urgent admission for new psychosis.

Recent research has identified genes that increase susceptibility to schizophrenia. One in particular has a variant allele that makes people especially susceptible to schizophrenia should they use cannabis: a classic gene-plus-environment interaction. If Mitchell has one or two copies of the variant, he may be the first child murderer we can recognise as genetically prone rather than inherently evil.

A developing or immature brain may be susceptible in other ways. Anecdotal reports link cannabis with suicide, paranoia (and the perceived need to carry knives), mob aggression by schoolkids, and fatal road accidents.

Most worrying is the unknown effect on the human foetal brain. Behavioural disorders and cognitive damage have been reported in children exposed in utero to cannabis. Brain imaging of young adults exposed prenatally to marijuana showed persistent changes and performance deficits compared with controls. Very recent Swedish studies show striking changes in the foetal brain's limbic system, which regulates emotional behaviour. The developing brain is clearly more vulnerable to drug damage than the adult.

Other evidence that should be heeded is that cannabis alters reward-pathways in the brain, perhaps speeding the progression to other drugs of addiction. There are also medical reports that describe cannabis causing transient ischaemic attacks (mini-strokes) in young adults and even a full-blown cerebral infarct (stroke) in a 15-year-old who used the drug heavily. Finally, among smokers of hash lung cancer could be a long-term risk, possibly increased by tobacco use.

Humans are a risk-taking species, but let's not continue to call cannabis a "safe" drug. It should be reclassified as Class A for the under-25s and pregnant women, and dealers should suffer severe penalties.

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