U.S. health regulators are reviewing the effectiveness in some patients of the widely used blood thinner Plavix, made by Sanofi-Aventis SA and Bristol-Myers Squibb Co.

The move comes after several studies found the blockbuster drug did not work as well in certain people possibly because patients metabolized it differently depending on their genetic makeup or their other medications, the U.S. Food and Drug Administration said on Monday.

Sanofi and Bristol have agreed to conduct studies on genetic factors and drug interactions with Plavix, also known as clopidogrel bisulfate, the FDA said in a statement on its website. "This information should lead to a better understanding about how to optimize the use of clopidogrel," it said.

Representatives for the two drugmakers could not be immediately reached for comment.

While some reports have suggested a possible interaction that makes Plavix less effective, others have not found such an issue, the FDA said.

One type of drug that is often prescribed for patients also taking Plavix are heartburn drugs such as AstraZeneca PLC's Nexium and Prilosec and other proton pump inhibitors, the agency said.

The FDA said doctors and patients should continue to use Plavix as directed until further information is available. Doctors should reevaluate the use of some heartburn drugs, including over-the-counter versions, in patients taking Plavix, the FDA added.