Teva has brought a generic version of Bayer's YAZ birth control pill to U.S. markets earlier than the drugmakers had previously agreed, prompting Bayer to file a law suit.

Israel's Teva, the world's largest generics maker, said on Tuesday it launched the contraceptive under the brand name Gianvi, followed by a Bayer spokesman saying that the German drugmaker would sue Teva for patent infringement.

Teva said YAZ generated sales of $782 million in the U.S. in 2009, citing data from market researcher IMS, and adding it had won a 180-day period of marketing exclusivity. Bayer had previously lost a legal tussle with Teva over the right to sell copycat versions of its bestselling Yasmin and YAZ pills in the U.S.

But Bayer in 2008 agreed to supply the pills to Teva in exchange for royalty payments, cutting its losses from cheaper competition. As part of that deal, Teva would not have launched YAZ in the U.S. before July 2011, unless other generics makers managed to enter the market earlier.

"(The launch) comes as a surprise as we had previously expected Teva to launch generic Yaz in line with its agreement for an authorized generic from July 2011," analysts at Deutsche Bank said in a note to investors.

Bayer could lose €200 million in 2010 sales and €250 million in 2011, the brokerage added. Bayer had also taken Watson and Novartis Sandoz to court for trying to sell generic versions of YAZ in the U.S.

In the global market for hormonal birth-control pills, Bayer also competes with Johnson & Johnson of the U.S. and Ireland-based Warner Chilcott. The product group comprising Yasmin, YAZ and Yasminelle, all based on the hormones ethinylestradiol (estrogen) and drospirenone, are Bayer's best-selling pharmaceutical product, generating €1.3 billion in revenues last year.