PARIS (AP) -- The U.S. Justice Department is investigating drug maker Sanofi's disclosures to the Food and Drug Administration about different responses to its blockbuster blood thinner Plavix.

The French company said in a filing with the Securities and Exchange Commission last week that it learned in June about the investigation.

It only said the investigation centered on "the variability of response to Plavix." The company did not immediately respond to requests to give more details on the investigation.

In 2010, the FDA added a black box -- its strongest form of warning -- to the label for Plavix. At the time, the watchdog said certain patients with a genetic variation cannot metabolize the drug, putting them at increased risk for heart attack and stroke.

Before it got U.S. generic competition last May, Plavix was the No. 2 drug in the world by revenue, with annual sales of around $9 billion in 2011.

Sanofi says it is cooperating with the investigation into Plavix, which is jointly marketed with U.S.-based Bristol-Myers Squibb. Bristol-Myers Squibb spokeswoman Jennifer Fron Mauer said the company had no comment on the investigation.

Plavix is prescribed to heart disease patients to prevent dangerous blood clots, which can cause heart attack, stroke and death.

In order to work effectively, Plavix must be broken down by a particular liver enzyme. But the FDA says 2 to 14 percent of people in the U.S. have low levels of the enzyme, preventing them from successfully processing Plavix. The likelihood of being a non-responder varies by race, according to the FDA.

Copyright 2013 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.