The ill-tempered argument over cannabis use is an ideological dispute between those who regard it as an entirely safe herb and those who regard it as a devilish drug likely to drive one mad after a few puffs. Sadly, there is rarely an honest and open discussion on why people enjoy it, what level of consumption is safe, if adolescents are especially at risk and whether certain types of cannabis pose greater dangers.
Professor David Nutt, who was sacked as the Government’s drugs adviser, claims that cannabis is no more dangerous than alcohol or cigarettes. But is it sensible to equate even these two? There are no health benefits in smoking cigarettes, which is simply a manifestation of nicotine addiction. Most adults in Britain, however, drink alcohol in moderation, and suffer few adverse consequences. Yet most know that drinking a bottle of vodka a day is likely to harm their health, and few favour daily drinking from the age of 14.
There is a lot of evidence that smoking several joints a day impairs memory. Using brain imaging, cannabis can be seen to impair the function of the hippocampus, a structure essential to memory. If we drink heavily for an evening, we may remember little the next day. But alcohol is washed out of the system in 24 hours; cannabis can be detected a month later. No wonder the academic performance of school children who smoke it daily deteriorates; they have a chronic low-grade intoxication.
It remains disputed whether memory returns totally to normal after consumption ceases. In those who have smoked only for a few months or years, there is every hope of a return to normality in months. But whether a “stoner” addled by decades of consumption can regain all his faculties remains uncertain.
People with schizophrenia use more cannabis than the general population; that is undisputed. The critical question is whether cannabis use has helped to cause it. Evidence has been mounting steadily over the past six years that initially healthy people who use cannabis daily are more likely to develop paranoia and psychosis. Those with a family history of mental illness, with a suspicious or psychosis-prone personality and those who start cannabis use in early adolescence appear at greater risk.
It is not surprising that the link between cannabis and schizophrenia remains controversial. It took several decades for society to accept that smoking tobacco increases the risk for lung cancer. Now it is generally accepted. In his lecture last week Professor Nutt contrasted a 2.6-fold increase in the risk of psychosis from using cannabis with a twentyfold increase in the risk of lung cancer if one smokes cigarettes. Unfortunately he was not comparing like with like here. The twentyfold increased risk does not come just by being a smoker but by being a long-term heavy smoker. For cannabis, the risk of schizophrenia rises about sixfold if one is a long-term heavy user.
Nevertheless, 90 per cent of daily users will not develop schizophrenia, just as most daily smokers will not die of lung cancer. But even among those who never come near a psychiatrist, daily cannabis users are more likely to be unsuccessful in their careers, to have broken marriages and to suffer minor psychological problems such as anxiety and depression. For teenagers and their families the consequences of cannabis smoking may never register in medical statistics, yet be devastating in their impact on family life.
In 2007 the Advisory Council on Misuse of Drugs concluded that street cannabis was getting more potent, a view confirmed by two large surveys. These showed that the concentration of THC, its main active ingredient, had increased from about 4 per cent in 1970s resin to 14-18 per cent in present day skunk.
Giving THC to normal volunteers in a laboratory setting induces psychotic symptoms to a varying extent, depending on the dose. Cannabis is a complex substance and THC is not the only constituent. Another is cannabidiol (CBD), which does not induce psychosis in studies; if anything it relieves anxiety, and may even have antipsychotic effects. Traditional cannabis resin contains equal amounts of THC and CBD, so some pro-psychotic effects of THC may be counterbalanced by the CBD. In skunk, not only has the concentration of THC greatly increased but the CBD has virtually disappeared, so it may carry a double whammy. Skunk’s ever-larger share of the street market is worrying.
There is little evidence that altering the legal classification of cannabis affects consumption. When the law was liberalised in 2004 cannabis consumption was falling in most of Western Europe, and the UK has followed the general trend.
Two Western European countries with markedly different policies have less of a problem than the UK. The Swedes adopt a prohibitionist line and only about 8 per cent of adolescents have smoked cannabis. The Dutch have a very liberal policy and 28 per cent of adolescents use cannabis. Both have lower consumption than the UK where 38 per cent of 16-year-olds have used it. Teenagers starting to use cannabis do not debate its exact classification; many do not even think it is a drug and few have any knowledge of its hazards. What does appear to have an effect is knowledge about the risks of heavy consumption.
Rather than arguing whether cannabis should be a Class B or C drug, politicians would be better to support a big educational campaign on the properties and constituents of different types of cannabis and the risks associated with heavy use.
Robin Murray, professor of Psychiatric Research at the Institute of Psychiatry, King’s College London.