This time, it’s in Europe. Switzerland’s Schweizerhall expects to market a lower-priced version of the widely used blood thinner within the month, and the news sent Sanofi-Aventis shares plummeting.

“The German authorities have basically finished the registration procedure and we’re now just waiting for the paperwork,” Luzi Andreas von Bidder, Schweizerhall’s chairman, tells Bloomberg News. The drugmaker is working with a “major generics company” and expects to start selling the product, called clopidogrel, this quarter. Sandoz and Ratiopharm will also market a generic Plavix, Dow Jones reports.

This is like a deja vu for Sanofi. You may recall that just two years ago, a generic Plavix was briefly sold by Canada’s Apotex, slashing some $1.75 billion in sales off the top line at Bristol-Myers Squibb, which jointly markets the drug with Sanofi in the US. Sanofi, which says the Plavix patent expires in 2013, is evaluating its legal options, but investors are concerned.

“We did not think at all Plavix hegemony would have been disputed before 2013,” Richelieu Finance analyst Alexandre Iatrides tells Bloomberg. “There are fears for Plavix competition all over Europe. In that case, it could cost 10 percent of Sanofi’s net income.”

Last year, Plavix generated $3.74 billion for Sanofi, making the blood thinner the drugmaker’s second-best-selling med behind the Lovenox clotting treatment. As for Bristol-Myers, Deutsche Bank analyst Barbara Ryan notes in an investor bulletin this morning that the drugmaker co-markets Plavix in Germany, Greece and Spain and records a portion of those revenues. “In our view, the exposure is greatest to generic erosion in Germany, but the earnings impact…appears fairly small,” she writes.