The FDA yesterday updated the label on a widely used blood thinner, warfarin, to include information about how a patient's genes will predict response to the drug. Patients with certain genetic variations should receive a lower dose of the drug, according to the added text.

Although a handful of drugs already note such genetic information on their labels, most of these are cancer drugs, which are used by a well-circumscribed set of patients. Warfarin is much more widely prescribed - about two million people start taking it each year, according to the FDA press release. Although the drug is notorious for its dosing difficulties, it has been around for more than 50 years, and doctors are used to managing dosing by trial and error.

Though FDA commissioner Andrew C. von Eschenbach in the agency's statement calls the label change "one step in the FDA's commitment to personalized medicine ," what exactly doctors and patients are supposed to do with this genetic information is not entirely clear. A front-page article in the Wall Street Journal yesterday noted that although genetic tests can identify who has certain genetic variants, researchers have heatedly debated whether enough is known about these variations to determine what they mean for the patient.

One clinical researcher quoted in the article, Ann Wittkowsky of the University of Washington, said that "It would be irresponsible and potentially harmful to suggest that testing be used, or even mentioned, in the label," adding that the testing is "fascinating science, but it is not yet ready for prime time."