Why cannabis doesn't kill

When I read last week that as many as 30,000 deaths a year in Britain could be caused by smoking cannabis, I reacted in a most uncustomary way for a mild-mannered scientist. I was infuriated. As someone acknowledged throughout the world as a specialist in how drugs affect the brain, I know a great deal about cannabis, and I know that the facts are not there to stand up such an outrageous statement.

The main problem is that cannabis has been classified incorrectly for nearly 50 years as being an extremely dangerous drug, but it doesn't fit that level of hazard. I'm not saying it's completely safe - no drug is completely safe, but as recreational drugs go, it's one of the safer ones. You can't overdose on cannabis, but you can certainly overdose on heroin, and even on alcohol.

It is headline-grabbing rubbish to suggest that cannabis could be responsible for so many deaths in future. To be sure, it makes a great story, but when you look at the arithmetic, it doesn't add up.

The original British Medical Journal article suggested that the chemicals which are given off when cannabis is smoked could mean that users would succumb to the same diseases that affect tobacco smokers. But in my view, it overlooks several significant facts.

Cannabis smoke does contain many of the same poisonous chemicals that you find in cigarette smoke, and cannabis smokers draw more tar into their lungs than cigarette smokers because they tend to inhale more deeply, and then hold their breath.

But to end up with as much tar in their lungs as a 20-a-day cigarette smoker, a cannabis user would have to smoke four or five joints a day, every day of the week.

But most of the million or so in this country who smoke cannabis do so at the weekend (I believe the article's figure of 3.2 million users is again wide of the mark) and the great majority quit when they reach their thirties. If the risks of smoking cannabis equate to those of tobacco, those who quit before they are 35 only have a slightly increased risk of lung cancer, just one or two per cent.

Yes, cannabis smoke has some harmful effects. It irritates the lungs just as much as tobacco, and there is some evidence that it causes a nasty cough, which can lead to bronchitis, but to say that this leads to lung cancer is a huge leap in the dark.

You can't extrapolate like that because the hard evidence does not exist. Of course, you could say that 50 years ago we didn't know that cigarette smoke was so harmful, but to put a number on the risk of cannabis at this stage, with random figures, is scaremongering. The report suggested that tetrahydrocannabinol - the nicotine equivalent in cannabis - can increase by more than four times the chances of a heart attack within an hour of taking it, and also mentioned that most cannabis sold on the UK black market is now 10 times stronger than it was 20 years ago.

These things may be true, but it is also a fact that in Britain, no drug-related deaths due to cannabis have been reported for many years. So you simply cannot conclude that smoking cannabis is likely to give you a heart attack.

Another scaremongering tactic from the anti-cannabis brigade is that regular use means a higher risk of mental illness, such as schizophrenia and depression. Certainly, studies have been published which show an "association", but that doesn't prove cause and effect.

It doesn't mean that one thing automatically leads to the other. That is not the way scientists should conduct experiments, nor draw conclusions in print.

I am not approving the fact that so many young people smoke cannabis, but we must learn to be more grown-up about the way we debate the subject. We have to look at the facts in a more dispassionate way.

And, at least in the sense that the issue can now be debated openly, that has begun to happen. No one would discuss cannabis, even relatively recently. When I advised the House of Lords committee five years ago that cannabis was not as damaging as, for instance, regular smoking or drinking, no one wanted to know about our findings. Now, things have changed. A number of serious studies have been done, which is moving the debate in the right direction.

It is never possible for a scientist to say that anything is totally safe. But, at the end of the day, scaremongering does science - and the public - a great disservice. Cannabis is simply not as dangerous as it is being made out to be.

Professor Les Iversen of the Department of Pharmacology at Oxford University is the author of The Science of Marijuana, published by Oxford University Press.