Pharmaceutical companies love dish about doctors and patients almost as much as they love drug profits. One source of information they like to plunder to expand their markets is doctors’ prescribing histories. These “who,” “what,” “why” reports are one component of so-called “data mining” that has gotten much attention lately as a sometimes sneaky way to unearth potentially sensitive information.

The companies buy the reports from prescription drug intermediary (PDI) agencies that, as explained in an article in the New England Journal of Medicine, collect the prescription records from pharmacies and link them to physician information purchased from the American Medical Association.

Pharma sales representatives crunch the numbers in order to refine their sales pitches when they visit doctors’ offices. Critics of this process, known as “detailing,” claim that it...