The Food and Drug Administration say the stomach-soothing drug Prilosec lowers the effectiveness of Plavix by half.
Plavix is marketed by Sanofi-Aventis and Bristol-Myers Squibb. It 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.
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