Federal health officials said today a popular variety of heartburn medications can interfere with the blood thinner Plavix, a drug taken by millions of Americans to reduce risks of heart attack and stroke.

The Food and Drug Administration said the stomach-soothing drugs Prilosec and Nexium cut in half the blood-thinning effect of Plavix, known generically as clopidogrel.

Regulators said the key ingredient in the heartburn medications blocks an enzyme the body needs to break down Plavix, muting the drug's full effect. Procter & Gamble's Prilosec is the over-the-counter version of AstraZeneca's Nexium, which was first approved in 2001.

Plavix is marketed by Sanofi-Aventis and Bristol-Myers Squibb. With global sales of $8.6 billion last year, it's the world's second-best selling drug behind Pfizer's cholesterol drug Lipitor.

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

The FDA says patients who need to reduce their acid should take drugs from the H-2 blocker family, which include Johnson & Johnson's Mylanta and Boehringer Ingelheim's Zantac. FDA scientists say there is no evidence those drugs interfere with Plavix's blood clotting.