Shares of French drug maker Sanofi-Aventis (NYSE: SNY) have been tumbling more than 5% in morning trading on news that a Swiss drug maker said it expects to receive approval to sell a generic version of Sanofi's anti-clotting agent Plavix.

History is repeating itself. After facing generic competition in the United States to its second-biggest product in 2006, Sanofi-Aventis is now dealing with a similar threat in Europe. Competition concerns came after Switzerland's Schweizerhall Holding AG announced it would launch a copy of the Plavix blood thinner that could be bought for a lower price. Schweizerhall said it expects German regulators to approve its generic version of Plavix, called clopidogrel.

Sanofi-Aventis's fears about generic competition are justified as the company had to fight against a similar situation less than a year ago. Back in 2006, Bristol-Myers Squibb Co. (NYSE: BMY), which develops the product with Sanofi, saw a big plunge in its sales after Canadian generics company Apotex Inc. launched a cut-price copy of the drug.