The following chapters deal with the adverse health effects of cannabis. We have used the following criteria in making causal inferences about the adverse health effects of cannabis.

Making causal inferences

Standard criteria for making causal inferences (e.g. Hall, 1987) require: that there is an association between cannabis use and an adverse health outcome; that chance is an unlikely explanation of the association; that cannabis use preceded the health outcome; and that plausible alternative explanations of the association can be excluded.

Evidence of association

Evidence of an association between cannabis use and a health outcome is provided by the observation of a relationship between cannabis use and the outcome in a case-control, cross-sectional, cohort, or experimental study. These study designs differ in the ease and expense with which they can be enacted and in the strength of the inference that they warrant.

Excluding chance

Evidence is required that chance is an unlikely explanation of any relationship observed between cannabis use and a health outcome. ‘Unlikely to arise by chance’ is conventionally taken to mean that it is an event that would occur less than 5% of the time. In the biomedical sciences, the role of chance is evaluated by constructing a 95% confidence interval around the sample value of a measure of association, such as a correlation coefficient, an odds ratio or a relative risk (Altman et al., 2000). A confidence interval provides a range of values of the measure of association that are consistent at the 95% level of confidence with the value observed in the sample. If the confidence interval does not include the value consistent with no relationship, then one is able to infer that there is an association between cannabis use and the adverse health effect.

Ascertaining temporal order

If cannabis use is a cause of an adverse health effect then cannabis use should precede the health effect. Cross-sectional and case-control studies which assess cannabis use and health status at the same time often do not enable us to decide which came first, the cannabis use or the health outcome. The strongest evidence that cannabis use precedes the health effect comes from a cohort study in which the researcher observes that cannabis use precedes the health effect or an experiment in which the experimenter ensures by design that it does so.