Sanofi (NASDAQ:SNY) disclosed in a filing with the Securities and Exchange Commission last week that the U.S. Department of Justice is investigating disclosures to the Food and Drug Administration about how its blood thinner Plavix works in certain patients. Sanofi markets the drug with Bristol-Myers Squibb (NYSE:BMY).

The French drugmaker was light on details, simply disclosing that it learned of the investigation last June and that it's "cooperating with the U.S. Department of Justice in this matter."

In 2010, Plavix's label was changed to note that it was less effective in patients with a genetic variation that changed how they metabolized Plavix. According to The Wall Street Journal, U.S. state attorneys general in Mississippi and West Virginia sued Bristol-Myers and Sanofi last year claiming that the companies knew or should have known about the issue as early as 2003. According to WSJ, the lawsuits claim the companies didn't disclose the issue because it would have hampered sales in patients with the variation.

Plavix went off patent last year, so the investigation isn't likely to hurt the already substantially reduced sales, but could potentially result in a fine for Sanofi and/or Bristol-Myers.