(AP) -- The Food and Drug Administration is adding its strongest warning to the label for Plavix after reports that some patients cannot process the blood thinning drug.

The FDA says certain patients with a genetic variation cannot metabolize the drug, putting them at increased risk for heart attack and stroke.

Patients can determine if they don't respond to Plavix by taking a genetic test. The FDA recommends non-responding patients take other blood thinners, such as aspirin.

The new Plavix warning will appear within a black box, the FDA's most severe warning.

Plavix is marketed by Sanofi-Aventis and Bristol-Myers Squibb. With global sales of $8.6 billion in 2008, it was the world's second-best selling drug behind Pfizer's cholesterol drug Lipitor.