Doctors say that a drug “works” if, in comparison with the control arm of a clinical trial, significantly more people in the treatment arm respond. Unfortunately, this is a naive oversimplification, and it breeds complacency among patients and physicians alike.

Criticisms of this perspective have been lodged before. One is that researchers often pick outcomes that are not patient centred—patients do not care if a tumour shows “shrinkage upon radiological visualisation” but rather whether they are in less pain. Another criticism is that when side effects of drugs are factored in, many patients do not think that a drug works very well at all, even as the doctor or drug company extols its virtues; drop-out rates in the active agent arm of trials often exceed those of the placebo arm, providing evidence of patients’ distaste for the overall effects of a drug.

But one problem that has received far less attention is that when patients say a drug “works” they typically mean something quite different from what doctors mean. Patients mean …