The evidence also suggests that the consumption of cannabis, especially in high quantities, is responsible for many traffic accidents. But while the relationship between the level of alcohol in the blood and incapacity to drive (or operate other machinery) is relatively clear and straightforward, it is much less so in the case of cannabis. This is because cannabis remains detectable for a very long time in the body, and the relationship between level and effect is a much more complex one. This means that any law attempting to prohibit driving under the influence of cannabis would have to be either too lenient or too draconian, and would certainly create immense legal difficulties.

Sir Richard Branson referred in making his case to the experience of Prohibition in the United States. This history is often misunderstood and the analogies drawn are false. It is often claimed that the murder rate in the United States increased during Prohibition, and so it did: but it increased at an even faster rate in a similar period of time beforehand. Al Capone was called Scarface because of the injury he received during his criminal activities before Prohibition came into force. Furthermore, from the public health point of view, Prohibition was a great success, at least as far as the prevention of alcohol-caused diseases was concerned. This does not by itself justify it: there are other goals in life other than those of public health. But it is not true that it was a failure from that aspect. Furthermore, it is important to remember that we do not consider the law against murder a failure simply because, year after year, there continue to be murders and that therefore the “war” against murder has been lost. No law achieves exactly and only its ostensible purpose.

We should be wary of applying the experience of other countries too directly to our own, however. For example, relaxation of the drug laws in Portugal had been followed by only a relatively minor increase in consumption. But in Britain, the relaxation of the licensing laws led to a vast increase in public drunkenness and alcohol-related problems.

Genies are often difficult to put back into bottles.