More than any other drug, cannabis and its effects have been studied and debated over the past 10 years. But serious questions remain as to who is most vulnerable to the damage which can be caused by regular use of the drug.
On Friday, the Home Secretary, Charles Clarke, took the unprecedented step of ordering a review of the evidence about the health consequences of heavy cannabis use.
Clarke wrote to the Advisory Council on the Misuse of Drugs: ‘I want to be clear what influence the evidence presented within these studies has on the overall assessment of the classification of cannabis.’
It was only 15 months ago that Clarke’s predecessor, David Blunkett, reclassified cannabis from Class B to Class C, meaning it was no longer an arrestable offence to possess it. Cannabis is smoked by up to four million people in Britain every week, mostly on a casual basis.
Blunkett’s move to downgrade cannabis was welcomed by many, including this newspaper, as a sign of a more grown-up attitude to drugs in general.
But for the past 10 years there has been evidence that the drug could have profoundly damaging effects on the brains of some users. The first study indicating it could be harmful came out of Sweden in the early Nineties and was disputed. Since then six major studies following users over a number of years have linked cannabis use to the development of psychotic symptoms. More evidence has since emerged that some people’s genetic make-up may predispose them to be particularly vulnerable.
A study from New Zealand published earlier this month in the medical journal Addiction suggested ‘a clear increase in rates of psychotic symptoms after the start of regular use’. The study, over 25 years, followed 1,055 people born in 1977.
Some fear the issue is being polarised. Professor David Fergusson, of the Christchurch School of Medicine and Health Sciences, and lead researcher of the New Zealand study, said debate was split between those who believe cannabis causes terrible harm and those who think it does not and should be legalised. ‘I think the truth lies somewhere in the middle,’ he added.
The other major study to have influenced Clarke’s decision was by Professor Jim van Os at Maastricht University, in the Netherlands, in December. It concluded that use of the drug ‘moderately increases’ the risk of psychotic symptoms in young people but has ‘a much stronger effect in those with evidence of predisposition for psychosis’.
There is a growing belief within the medical community that people who have a genetic risk of mental illness will be affected far more by cannabis than those who do not.
Cannabis resin contains the cannabinoid tetrahydrocannabinol, or THC, which produces a sense of euphoria and an apparent heightening of all the senses, especially vision. It slows the reflexes and gives an illusory sense of deep philosophical conviction, and sometimes the feeling of omniscience.
It does have a medical value; for people with multiple sclerosis or nerve damage it is known to ease the pain and attempts are under way to make a tablet form of the active compound which could be licensed for clinical use.
What worries doctors is the increase over the past 10 years in the availability of super-strength cannabis, known as skunk, which contains a high level of THC and is far more likely to trigger panic attacks, manic depression or a condition known as cannabis-induced psychosis.
But measuring how much this occurs is difficult because most people tend to lie when asked by doctors about their drug habits.
Many parents are also worried about the effect of stronger strains of cannabis. Observer radio critic Sue Arnold has described how her son, now 24, fell ill after taking cannabis.
‘His first psychotic episode, as it’s called in medical terms, was triggered by smoking skunk. Since then he has been on anti-psychotic medication more or less regularly.’
Arnold herself has smoked cannabis in the past because she found it helped her vision - she is partially sighted. But her son has been mentally ill for three years, diagnosed as a paranoid schizophrenic and sectioned under the Mental Health Act twice, making her very sceptical about its effects.
While evidence is growing that daily use of cannabis can have a damaging effect on the brain, studies also suggest that occasional use will not be harmful for most.
The question now is whether the advisory council, headed by eminent scientist Michael Rawlins, will suggest a reclassification to protect those whose genes leave them vulnerable to the drug. Given that cannabis can cause potentially irreversible chemical changes in the brain, the review is as essential for public protection as it is for political reasons.