The Food and Drug Administration slapped its strongest warning on Plavix, a popular pill to prevent blood clots in people with heart disease. The problem: the drug may not work in some people.

Orlando, Fla. -- A new blood thinner proved better than Plavix, one of the world's top-selling drugs, at preventing heart problems after procedures to open clogged arteries, doctors reported today.

If you're taking Plavix to prevent a heart attack or stroke, lay off the Prilosec, the Food and Drug Administration says. It turns out that the popular heartburn pill Prilosec cuts the ...