This week, a study purportedly showed an association, or correlation, between people who became habitual cannabis users when they were 13 and people whose IQ dwindled between the ages of 13 and 38. It also suggested the lack of such correlation for those who first became habitual cannabis users when they were aged 18 or over. Accordingly, one is expected to infer that using cannabis in early adolescence has a detrimental effect on IQ, but doesn't if one begins using it in late adolescence.

These findings are likely to be mistakenly adopted by those who advocate continuing the prohibition of cannabis as a justification of their position. But assuming that such correlations are true, it is obviously important that cannabis use takes place within a framework where age limits may be imposed. I have been a dedicated supporter of the relegalising of cannabis since the mid-1960s. Neither I, nor any of the various pro-legalisation organisations with which I have been associated, have ever advocated that legalisation should not be accompanied by age limits and other controls. Age limits for a large number of activities are well entrenched and accepted by society, and tend to lie between the ages of 16 to 18.

But there are no age limits in a black market. Neither is there any other form of control. Prohibition is not control, and should not be equated as such. It is the abrogation of control leading to the unregulated peddling of adulterated substances outside the reach of the law. Apart from not beginning to achieve its aims, prohibition makes drugs artificially expensive and spawns an avalanche of acquisitive criminal behaviour.

Most drugs, for example cannabis, are either plants or are derived from plants, and most plants are very cheap. But as illegal drugs have to be sold on the black market, those who trade in them risk being sent to prison or fined. They charge a lot of money for their trouble. Some people cannot afford to buy the artificially expensive drugs they want to take and resort to theft. The illegal drug business has become so profitable that people have violent fights over territory in which to sell drugs. They can afford to buy guns. Guns are bad. Once a country is full of people with guns, the guns will never go away. Across the Middle East, South-east Asia, Africa and Latin America, civil war after civil war has been funded by the only strategy available to revolutionaries and counter-revolutionaries alike, the mass production of their traditional drugs for sale to the absurdly lucrative global market.

Prohibition increases any harm that might be caused by recreational drug use. A recreational drug, by virtue of its inherent chemical nature, cannot cause crime; it can only do so through interaction in a social context, which is prohibition and the consequent black market. Prohibition is a root cause of crime, violence and ill health. It would be difficult to construct, even if one deliberately contrived to do so, a policy more physically dangerous, more individually criminalising, or more socially destructive. Prohibition is a relatively recent social experiment, an extremely dangerous failure, and should be dismantled as soon as possible.

Legalisation does not require a set of laws enforceable by only the most totalitarian of police states. It is humane and helpful and no more condones drug misuse than a doctor prescribing a contraceptive condones promiscuity. As such, I welcome the findings of the study as demonstrating the increasingly urgent need to legalise cannabis.