What questions can a placebo answer?

The concept of clinical equipoise restricts the use of placebo controls in clinical trials when there already exists a proven effective treatment. Several critics of clinical equipoise have put forward alleged counter-examples to this restriction-describing instances of ethical placebo-controlled trials that apparently violate clinical equipoise. In this essay, we respond to these examples and show that clinical equipoise is not as restrictive of placebos as these authors assume. We argue that a subtler appreciation for clinical equipoise-in particular the distinction between de facto and de jure interpretations of the concept-allows the concept to explain when and why a placebo control may be necessary to answer a question of clinical importance.