Despite controversy, the FDA today is taking a big step toward using genetics to determine dosing of the med, which also goes by the brand name Coumadin. The agency is now saying the label for the Warfarin blood thinner will say that a lower initial warfarin dose “should be considered for patients with certain genetic variations.” (Here’s the complete labeling info).

“The labeling change highlights the opportunity for healthcare providers to use genetic tests to improve their initial estimate of what is a reasonable warfarin dose for individual patients. Testing may help optimize the use of warfarin and lower the risk of bleeding complications from the drug,” the FDA says in a statement. “These labeling updates are based on an analysis of recent studies that found people respond to the drug differently based, in part, on whether they have variations of certain genes.”

The warfarin tests have also led to a clash between the FDA and some docs, The Wall Street Journal reports (subscription required). Some specialists say testing hasn’t been proved to reduce the risks of the drug. They fear patients who don’t get the tests and run into trouble will sue docs, citing the drug’s label. While Medicare covers the tests, which generally cost between $300 and $500, major insurers such as Aetna, WellPoint and Cigna don’t. Insurers say they need more evidence about the benefits.

“It would be irresponsible and potentially harmful to suggest that testing be used, or even mentioned, in the label,” University of Washington professor Ann Wittkowsky tells the paper. “It is fascinating science, but it is not yet ready for prime time.”

“Although genetic testing can currently identify who has these genetic variants, more studies are needed to explore the precise starting dose for these patients,” says Larry Lesko, who directs the FDA’s Office of Clinical Pharmacology. “FDA has been working with other government agencies and organizations to develop such studies under the auspices of our three-year-old Critical Path Initiative, which addresses the challenges of moving promising medical products from discovery to patient use.”