Summary Box: FDA puts black box warning on Plavix
PLAVIX PROBLEM: The Food and Drug Administration is adding its strongest warning to the label for Plavix, cautioning that some patients do not respond to the blockbuster blood thinner.
GENETIC CAUSE: A small percentage of people have a genetic variation that prevents them from making an enzyme needed to break down Plavix.
BLOCKBUSTER: 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. Experts say the new FDA warning could push more doctors to prescribe Effient, a competing blood thinner launched by Eli Lilly last summer.