Amsterdam, famous for its canals and artists, is now, for far too many visitors, memorable mainly for its stag parties and coffee houses. The city has learnt its lesson. Fewer coffee houses are now being licensed and the many that still flourish are now more tightly regulated.

The British approach to cannabis, meanwhile, is still clouded by the lasting influence of the 1968 generation, who revered youth, whatever the cost, and were frightened to express an honest opinion about cannabis lest it sound old-fashioned, anachronistic or even racist.

The Swedes and the Dutch, whom few would consider quaint or anachronistic, have adopted a realistic approach to cannabis based on its medical side-effects. Conversely, Britain tackles the cannabis problem by producing impotent reports prepared by organisations whose members have been selected by the Government. The latest report on cannabis by the parliamentary Science and Technology Select Committee criticises earlier contributions to the debate on drugs from the police and the Government’s own Advisory Council for the Misuse of Drugs (ACMD). As its recommendations were dressed up in parliamentary language, it sounds less condemning of the present system, by which the Government derives its information from an advisory council that it appointed itself. Although allegedly peopled by experts, the council previously contained no pharmacological toxicologist, no neurologist nor any research biologist.

The better known psychiatrists who have spent a lifetime treating the psychoses that are related to cannabis-taking were not on the Government’s advisory council. The Government admitted that they had deliberately excluded members with known strong moral positions about drug relaxation (but not those with strong moral views about liberalisation). To have excluded detached, realistic, and genuinely independent opinions from the doctors who would be unlikely to have moral views but who would have seen the effects of cannabis can only have been destructive.

The select committee states that it intends to support a rational classification on drugs according to their ability to cause physical harm, and to induce dependence and impact on families, communities and societies, which sounds praiseworthy — but these are subjective evaluations and it will be difficult, if not impossible, to implement them.

It is easier to assess the damage that cannabis does to an individual, and the impact that this has on a patient and family than it is to judge its more nebulous influence on society and communities.

Research has now indicated, and is accepted by most medical experts, that an unexpectedly large proportion, possibly up to 25 per cent, of people may carry genes that make them vulnerable to cannabis, a vulnerability that may lead to psychotic breakdown. Cannabis can be the trigger that changes a brilliant, but eccentric and even bizarre, personality into someone who is so deranged as to be disabled.

An example of this effect was a young graduate who several years ago consulted me regularly. There was no doubt that she had a personality of the type that would make her vulnerable to the effects of cannabis. Equally, there was no doubt that the trigger for her breakdown had been cannabis.

Most doctors see many patients of this sort, and regret that had the patient been young 60 years earlier, he or she could well have been no more than eccentric and might well have had a rewarding and interesting career. What made a fairly routine case history different was the way in which her father’s response to his daughter’s diagnosis so underlined the select committee’s concern on the effect of cannabis on the family. His forcibly expressed opinion was that his daughter’s diagnosis of psychosis was a disappointment, as he had hoped that she had a cerebral tumour, even though this would lead to her death.

Many of the opinions of members of the Government’s advisory council on cannabis are on record. Those that suggest that cannabis is relatively harmless or even, as one member said, that there had been no deaths to date caused by the use of cannabis, are hard to explain as nearly 20 per cent of people who suffer from a chronic or recurrent psychosis commit suicide.

The use of cannabis is also being increasingly blamed for road accidents, acute heart problems and some malignancies. It is only necessary to compare the life expectation of someone with a psychosis with those of similar age and background without a psychosis to understand that cannabis takes years off your life.

Equally, if not more, important to the individual and society is the effect that cannabis has on everyday personality traits: apathy, loss of drive and effectiveness, emotional escapism leading to a misplaced enthusiasm for the mystical and esoteric, inadequate insight and poor judgment of both their own and other people’s abilities and actions.

Not to mention the effect that cannabis has on potency and fertility, and its ability to harm the embryo.