However, some heart experts are skeptical, saying that patients taking heartburn medications may have other underlying health problems. The data caused researchers to wonder if the heartburn drugs might be interfering with a liver enzyme required to metabolize Plavix.

And Paris-based Sanofi-Aventis SA, has been prescribed to more than 90 million people worldwide. Plavix, marketed by New York-based Bristol-Myers Squibb Co. The drug had global sales of $7.3 billion in 2007. The agency said both subgroups may have trouble metabolizing the world €™s second-best-selling drug. The agency is also investigating whether the drug, which prevents deadly blood clots and reduces the risk of stroke and heart attack, may be less effective in those with certain genetic backgrounds. Food and Drug Administration (FDA) are looking in to whether Plavix, a popular blood thinner used by millions, may be less effective in patients who also take certain heartburn medications. In November, researchers discovered that Plavix users who also took popular prescription heartburn medications, such as Nexium, significantly increased their chances of being hospitalized for stroke, heart attack or chest pain.

FDA Reviewing Plavix Concerns Officials with the U.S.