Although it is often vigorously contested and has several different formulations, the precautionary principle has in recent decades guided environmental policy making in the face of scientific uncertainty. Originating from a criticism of traditional risk assessment, the key element of the precautionary principle is the justification for acting in the face of uncertain knowledge about risks. In the light of its growing invocation in various areas that are related to public health and recently in relation to drug safety issues, this article presents an introductory review of the main elements of the precautionary principle and some arguments conveyed by its advocates and opponents. A comparison of the characteristics of pharmaceutical risk management and environmental policy making (i.e. the setting within which the precautionary principle evolved), indicates that several important differences exist. If believed to be of relevance, in order to avoid arbitrary and unpredictable decision making, both the interpretation and possible application of the precautionary principle need to be adapted to the conditions of pharmaceutical risk management.