THE MOST nostalgic of the 1968 emotionally scarred generation still believe that there is no association between cannabis and psychosis. Some will even suggest that cannabis smoking is preferable to drinking alcohol. This week a leading police officer advanced the theory on radio that crushing cannabis smoking in a district was detrimental to both the individual’s health and to the law and order within the community. He said that in his experience the amount of cocaine taken in any one area is inversely proportional to the amount of cannabis used. Come down hard on the cannabis users and the result could be that there would be a cocaine problem.

Doctors who have been dealing with the ill effects of cannabis smoking were therefore relieved to read in the BMJ about a recent study of cannabis use, and its ability to precipitate psychotic symptoms in young people, especially if they had already shown symptoms which suggested a predisposition to psychiatric problems. Most medical practices have had patients who were young, bright and amusingly bizarre who appeared to have a good future awaiting them, only to have it dashed once they started to smoke marijuana.

There was a relationship between the amount of cannabis smoked and the likelihood that the user would develop psychotic symptoms. The more someone smoked the greater the likelihood of psychotic symptoms. These symptoms are not always so serious as to be described as a psychotic breakdown, but even lesser symptoms can affect the ability of a young person to do their job properly or to make good social relationships.

The research published in the BMJ was carried out by psychiatrists in Maastricht in the Netherlands. They took great trouble to adjust the findings for any confounding factors, such as concurrent use of alcohol, cigarettes, or other drugs, which might have given a bias to results.

The survey not only clearly demonstrated that exposure to cannabis during adolescence and young adulthood increased the risk of psychotic symptoms later in life but also confirmed other elements of the anecdotal evidence related by GPs. It showed, for instance, that as has always been suspected the association between smoking cannabis and the development of psychosis is much stronger if the smoker already has the type of personality that is associated with a predisposition to psychotic disease. This predisposition was assessed after a psychological study of the patient’s personality. Many doctors, who haven’t the skill and experience to do this, have noticed that a family history of a predisposition to psychotic diseases also increases the risk for cannabis smokers.

It has been my habit to tell young people in families where this tendency is obvious that smoking cannabis may be, and in fact is, undesirable for most students but it can be disastrous for those who carry these genes. The results of the Dutch study confirms anecdotal evidence and three earlier studies that cannabis may precipitate a serious psychotic breakdown and can lead to the emergence of less severe symptoms. These changes in personality can undermine someone ’s domestic life and career and lead to a lifetime of troubles.

Studies published five years ago also showed that the more cannabis smoked the more likely a patient would be to develop cancer. Regular light marijuana smoking more than doubled the likelihood of developing cancer of the head and neck (this includes the tongue and mouth). Daily users of cannabis who smoke more than one spliff a day increase the chances of developing one of these tumours by five times. If they both smoked cigarettes regularly and took cannabis the chances increased 36 times.

Marijuana appears as a stronger carcinogen than cigarettes, according to Professor Li Mao, from the M D Anderson Cancer Centre in Houston, Texas. Although cannabis may not be as detrimental to the heart and cardiovascular system as cocaine, researchers at Harvard found that the heart attack rate is five times higher for someone in the first hour after smoking a joint than it is at other times.