The health and psychological consequences of cannabis use - chapter 3

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3. Evidential principles

3.1 Issues in appraising health hazards

The evaluation of the health hazards of any drug is difficult for a variety of scientific and sociopolitical reasons. First, causal inferences about the effects of drugs on human health are not easy to make (ARF/WHO, 1981). Even inferences about the relatively direct and transient effects of acute drug use may be complicated by individual variability in response to a standard dose of a drug, and by the fact that serious adverse effects are relatively rare. Inference becomes more difficult the longer the interval between use and alleged ill effects: it takes time for such effects to develop, and it may take considerably longer for the research technology to be developed that enables these effects to be identified and confidently attributed to the drug use rather than some other factor (Institute of Medicine, 1982). In the case of chronic tobacco use, for example, it has taken over three hundred years to discover that it increases premature mortality from cancer, and heart disease. Moreover, new health hazards of tobacco use, such as passive smoking, continue to be discovered.

Second, in making causal inferences about drug use and its consequences there is a tension between the rigour and relevance of the evidence. The most rigorous evidence is provided by laboratory
investigations using experimental animals, or in vitro preparations of animal cells and micro-organisms in which well controlled drug doses are related to precisely measured biological outcomes. The relevance of such research to human disease, however, is often problematic. A great many inferences have to be made in linking the occurrence of specific biological effects in laboratory animals or cell cultures to the likely effects of the drug under existing patterns of human use.

Epidemiological studies of relationships between drug use and human disease have manifestly greater relevance to the appraisal of the health risks of human drug use, but this is purchased at the price of reduced rigour. Doses of drugs over periods of years are difficult to quantify in the best of circumstances. The vagaries of human memory which make quantification of consumption difficult in the case of tobacco and alcohol are magnified in the case of illicit drugs by the non-standard doses and contaminants in blackmarket drugs, and the reluctance of users to report illicit drug use. The fact that different patterns of drug use and other life-style factors are often correlated (e.g. alcohol and tobacco), makes attribution of ill-effects to particular drugs even more difficult (Task Force on Health Risk Assessment, 1986).

Third, appraisals of the hazards of recreational drug use are unavoidably affected by the societal approval or disapproval of the drug in question. As Room (1984) has observed, when evaluating the
impact of alcohol on non-industrialised societies, anthropologists have often engaged in problem deflation in response to the problem inflation of missionaries and colonial authorities. In our own
culture, the economic interests of tobacco and alcohol industries provide a potent reason for problem deflation with these drugs. Such problem deflationists often discount the adverse effects of alcohol
use, either by contesting the evidence for adverse effects, or by denying that there is a causal connection between alcohol use and particular adverse health effects.

Similar processes have been at work in the appraisal of the health effects of recreational cannabis use. The countercultural symbolism of cannabis use in the late 1960s has introduced a strong sociopolitical dimension to the debate about the adverse health effects of cannabis. Politically conservative opponents of cannabis use, for example, justify its continued prohibition by citing evidence of the personal and social harms of its use. When the evidence is uncertain, as it is with many of the alleged effects of chronic use, they resolve the uncertainty by assuming that the cannabis is unsafe until proven safe.
Complementary behaviour is exhibited by some proponents of decriminalisation. Evidence of harm is discounted or discredited, and uncertainties about the ill-effects of chronic cannabis use are resolved by demanding more and better evidence, arguing that until this uncertainty is resolved individuals should be allowed to exercise their free choice about whether or not they use the drug.

Such approaches to the appraisal of evidence have not always been consistently applied. Both sides of the debate would reject the application of their own approaches to the appraisal of cannabis to
the appraisal of the health hazards of alcohol, pesticides, herbicides, or chemical residues in food. While we do not claim to be unaffected by these processes, we will be as explicit as possible
about the evidential standards that we have used, and as even-handed as we can in their application.