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The Food and Drug Administration Friday placed a black-box warning—its most severe safety labeling—on the blood thinner Plavix. In a statement, it said that the drug could put certain patients at increased risk for heart attack and stroke.

Those who do not respond to the widely used blood thinner have a genetic variation and cannot metabolize the drug. The FDA recommends these patients take other blood thinners, such as aspirin. A genetic test can determine which patients have trouble metabolizing the drug.

Plavix, which is prescribed to heart disease patients to prevent dangerous blood clots, was the world’s second-best selling drug behind Pfizer’s cholesterol drug Lipitor in 2008. Sanofi-Aventis and Bristol-Myers Squibb market Plavix.