U. S. health regulators said they are investigating the effectiveness of the popular blood thinner Plavix which is the second-best-selling drug in the world, in some patients. The Food and Drug Administration is examining reports that link taking certain heartburn medications alongside Plavix to a decrease of the drugs beneficial action as well as examining the chances of a patient's genetic background interfering with the drugs effectiveness.

The U. S. Food and Drug Administration in a statement on its website said that Sanofi-Aventis SA and Bristol-Myers Squibb Co, manufacturers of the drug Plavix have agreed to conduct studies on genetics and drug interactions. The studies will take several months to complete.

The FDA has said doctors and patients should continue to use Plavix as directed until more information is available. They have recommended that doctors re-evaluate the use of certain heartburn drugs including over-the-counter versions, when to be taken by patients on Plavix.