The Big Business of Selling Prescription-Drug Records

Since the days of 19th century remedies such as castor oil laxatives and mercury syphilis tablets, pharmacists and patients have had a tacit understanding: Whatever you buy is confidential. No longer. Drugmakers and Internet companies are quietly joining forces to link pharmacy records with registrations at websites to target ads to people reflecting their health conditions and their prescription drugs.

In a process known as a matchback, third-party companies assign patients unique numerical codes based on their prescription-drug records. Websites use the same process to assign codes to registered users. Then databases can be linked—without names changing hands. That lets pharmaceutical companies identify groups who use a specific medicine and send them tailored Web ads.