The extension of the precautionary principle to the field of healthcare is the social response to two demands of the population: improved health safety and the inclusion of an informed public in the decision-making process. The necessary balance between cost (treatment-induced risk) and benefit (therapeutic effect) underlies all healthcare decisions. An underestimation or an overestimation of cost, i.e. risk, is equally harmful in public healthcare. A vaccination should be prescribed when its beneficial effect outweighs its inevitable risk. Mandatory vaccination, such as in the case of the Hepatitis B virus, is a health policy requiring some courage because those who benefit will never be aware of its positive effect while those who are victims of the risk could resort to litigation. Defense against such accusations requires an accurate assessment of risk and benefit, which underlines the importance of expertise. Even within the framework of the precautionary principle, it is impossible to act without knowledge, or at least a plausible estimation, of expected effects. Recent affairs (blood contamination, transmissible spongiform encephalitis by growth hormone, and new variant of Creutzfeldt-Jacob disease) illustrate that in such cases the precautionary principle would have had limited impact and it is only when enough knowledge was available that effective action could be taken. Likewise, in current debates concerning the possible risks of electromagnetic fields, cellular phones and radon, research efforts must be given priority. The general public understands intuitively the concept of cost and benefit. For example, the possible health risks of oral contraceptives and hormone replacement therapy were not ignored, but the public has judged that their advantages justify the risk. Estimating risk and benefit and finding a balance between risk and preventive measures could help avoid the main drawbacks of the precautionary principle, i.e. inaction and refusal of innovation, highly restrictive administrative procedures, and a waste of funds on the search for the utopian goal of zero risk. Other drawbacks are more insidious. The precautionary principle could contribute to a general feeling of anxiety and unease in the population. It could be used by campaigns to manipulate public opinion in favor of a particular commercial interest or ideology. Furthermore, practitioners and public policy makers could be led to make choices not dictated by a search for the optimal solution but rather a solution that would protect them from future accusations (the so-called umbrella phenomenon). On the international level, the precautionary principle must not be used to mask protectionism. Nevertheless, a clear advantage of the precautionary principle is that it requires decision-makers to explain the rationale behind their decisions, to quantify the risks, and to provide objective information. However, the physician must not be tempted to make patients sign documents certifying that they have been given all relevant information on his or her diagnosis and treatment. This example underlines the role of legal texts and jurisprudence in the application of the precautionary principle. Finally, the precautionary principle implies new obligations for the State. In the field of health and healthcare, the State must undertake actions based on fully open and undisguised decision-making and provide complete information to the public. A pplication of the precautionary principle requires much discernment because the final outcome can be beneficial or harmful, depending on the way it is implemented. The precautionary principle, and its applications, must be precise and detailed within a well-defined framework.