WASHINGTON (November 17, 2009)—The U.S. Food and Drug Administration says the popular heartburn medication Prilosec can block the blood thinning effect of Plavix, which is taken by millions of Americans to reduce risks of heart attack and stroke.

The Food and Drug Administration say the stomach-soothing drug Prilosec lowers the effectiveness of Plavix by half.

Plavix, which is marketed by Sanofi-Aventis and Bristol-Myers Squibb, is the world's second-best selling drug with global sales of $8.6 billion last year.

Because Plavix can upset the stomach, it is often prescribed with acid-blocking drugs.

The FDA says patients who need to reduce their stomach acid should take alternative drugs like Johnson & Johnson's Mylanta or Boehringer Ingelheim's Zantac.