Much of the fierce debate about cannabis in recent years has centred on how medically safe it is to use. But while questions such as "Does it cause long-term brain damage?" "Does it lead to harder drugs?" and "Does it induce schizophrenia?" are clearly important, they do not cover the whole story. A bigger issue is the loss of potential of our youngsters.
Research has shown that cannabis use can impair thinking, as shown by a drop in IQ and deficits in memory, vigilance and the capacity for complex mental arithmetic.
Any one of us who has occasionally over-indulged in alcohol might recognise similar symptoms, but because of the different way the psychoactive chemicals in cannabis are stored in the body, the effect on users' thinking can last up to a week.
Studies in flight simulators show that, in the hours following cannabis use, experienced pilots are over six times more likely to make errors which would take the aeroplane out of its designated airspace, with potentially dire consequences. Some impairment in performance persisted for five days.
What is particularly striking from this evidence is that the lasting effects of cannabis use are most obvious in complex tasks. Users may appear to be behaving normally, with no obvious signs of intoxication. However, give them a challenge which will stretch their thinking, and the cannabis taken some days before will limit their achievement. Thus young people can put in a passable performance while being kept from realising their full potential.
While the debate about the long-term medical effects of cannabis continues, at the very least we know that users can suffer from long-lasting problems with the ability to learn and remember new information. Even if a regular user allows weeks for the drug to clear their system, the effects on their brain may be permanent.
Cannabis may not kill users or cause long-term psychiatric problems for everyone, but the fact that it impairs thinking well beyond the time it is actually used leads to a significantly lower level of educational achievement. And, perhaps not surprisingly, there is a knock-on effect on income: further research shows that heavy users earn less in the long run.
In Every Child Matters, the Government set out the aspiration "for every child, whatever their background or their circumstances, to have the support they need: to be healthy, stay safe, enjoy and achieve, make a positive contribution and achieve economic well-being." Can we really afford to let cannabis stand in the way of that aim?
Baroness Susan Greenfield is professor of pharmacology at Oxford University and director of the Royal Institution of Britain
Iversen, L (2005) Long-term Effects of Exposure to Cannabis. Current Opinion in Pharmacology 5:69-72
McArdle, PA (2006) Cannabis Use by Children and Young People. Archives of Disease in Childhood 91:692-695
Cannabis and Flying.