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Food and Drug Administration officials said yesterday they are bringing to doctors' attention the potential usefulness of getting a patient's genetic profile before prescribing warfarin, one of the most widely used -- and most dangerous -- drugs on the market.

Variations in the activity of two genes can greatly increase the potency of warfarin, a pill that slows coagulation of the blood and that about 2 million Americans start taking each year. Many continue to take the drug for the rest of their lives.

While the FDA did not explicitly tell physicians to order genetic testing before prescribing warfarin, this is the first time the agency has provided data on the usefulness of such tests in the "label," or official instructions, of a popular drug.

"It shows that genomic medicine has moved into the mainstream," said Lawrence J. Lesko, of the agency's Center for Drug Evaluation and Research.

Warfarin, which is often sold under the trade name Coumadin, is taken by people who have had blood clots in their legs or lungs, who have suffered strokes while taking aspirin...