A recent systematic review of the ethics of randomised clinical trials shows that they are often justified on the basis of the uncertainty principle. 1 2 The central idea is that people contribute to posterity at no cost to themselves, if the “best” treatment is “unknown.” This idea has been used to describe the scientific case for trials and to guide informed consent when individuals are invited to participate. Two examples illustrate this. The United Kingdom's Central Office for Research Ethics Committees suggests the following wording for information leaflets given to the participants of trials: “Sometimes because we do not know which way of treating patients is best, we need to make comparisons.”3 Donovan et al recently described factors affecting recruitment to a randomised clinical trial of active monitoring, radiotherapy, and radical prostatectomy and “found it necessary to emphasise that recruiters must be genuinely uncertain about the best treatment, believe the patient to be suitable for all three treatments, and be confident in these beliefs.”4

In both cases the concept that the best treatment is unknown, which properly explains why a trial is worth doing, is carried over into the invitation to participate. Here I argue that such language, while providing the scientific and social rationale for trials, is inadequate—indeed misleading—when used to suggest that a patient might participate at no personal cost. This is because the words “best” and “unknown” are far too imprecise to …