The health and psychological consequences of cannabis use - chapter 8

THIS DOCUMENT HAS BEEN RESCINDED: Chapter 8.9 The limitations of anecdotal evidence

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8. The therapeutic effects of cannabinoids


8.9 The limitations of anecdotal evidence


Much of the case for the therapeutic uses of cannabinoids as other than anti-emetic agents depends upon anecdotal evidence from case histories. Such evidence has justifiably come to be distrusted as evidence of therapeutic effectiveness in clinical medicine, especially in the case of chronic conditions which have a fluctuating course of remission and exacerbation. In such diseases, it is difficult to exclude alternative explanations of any apparent relationship between the use of a drug (e.g. THC) and an improvement in a patient's condition. Among the alternative explanations that are most difficult to exclude in a single case or even a succession of single cases is simple coincidence: that is, there may be no relationship between the use of the drug and improvement; the apparent relationship between the two may have arisen because the use of the drug preceded an improvement in the patient's condition that would have occurred in its absence. This is especially likely to occur in a chronic condition with a fluctuating course. In addition, the well-known placebo effect which is observed in many conditions may explain the apparent benefits of a drug or other treatment. It is for these reasons that this review has relied upon evidence from controlled clinical trials in appraising the therapeutic uses of cannabinoids.

Grinspoon and Bakalar (1993) have attempted to defend anecdotal evidence of therapeutic efficacy of cannabinoids. They argue that a double standard has been used in the appraisal of the safety and efficacy of cannabinoids: anecdotal evidence of harm has been readily accepted while anecdotal evidence of benefit has been discounted. Although at first glance "double standards" may seem to describe the behaviour of the regulatory authorities, it is defensible to use different standards of proof when evaluating the benefits and the costs of therapeutic drugs. It is reasonable to err on the side of caution by requiring stronger evidence of benefit from putatively therapeutic drugs in order to ensure that the possible risks incurred by their therapeutic use do not outweigh their benefits. Moreover, this behaviour is not peculiar to the therapeutic appraisal of cannabinoids; it is standard practice in the therapeutic appraisal of all drugs. Medical practitioners are encouraged to report cases histories of possible adverse effects of prescribed drugs. Such reports are treated as a noisy but necessary way of detecting rare but serious side effects of drugs that have not been detected in clinical trials or animal studies.