Pediatrician Rupin Thakkar's first inkling that the pharmaceutical industry was peering over his shoulder ... came in a letter from a drug representative about the generic drops Thakkar prescribes to treat infectious pinkeye. In the letter, the salesperson wrote that Thakkar was causing his patients to miss more days of school than they would if he put them on Vigamox, a more expensive brand-name medicine made by Alcon Laboratories. My initial thought was 'How does she know what I'm prescribing?' " Thakkar said. "It feels intrusive ... I just feel strongly that medical encounters need to be private." He is not alone. Many doctors object to drugmakers' common practice of contracting with data-mining companies to track exactly which medicines physicians prescribe and in what quantities -- information marketers and salespeople use to fine-tune their efforts. The concerns are not merely about privacy. Proponents say using such detailed data for drug marketing serves mainly to influence physicians to prescribe more expensive medicines, not necessarily to provide the best treatment. "We don't like the practice, and we want it to stop," said Jean Silver-Isenstadt, executive director of the National Physicians Alliance. (Thakkar is on the group's board of directors.) "We think it's a contaminant to the doctor-patient relationship, and it's driving up costs." The American Medical Association makes millions of dollars each year by helping data-mining companies link prescribing data to individual physicians. It does so by licensing access to the AMA Physician Masterfile, a database containing names, birth dates, educational background, specialties and addresses for more than 800,000 doctors.