By Mantik Kusjanto

FRANKFURT (Reuters) - Shares in Bayer (BAYG.DE: Quote, Profile, Research) slumped as much as 5.6 percent on Tuesday after a U.S. court voided a key patent on its Yasmin oral contraceptive drug, prompting the German group to tweak its healthcare margin goal.

The court ruling capped years of legal wrangling and paved the way for Barr Pharmaceuticals Inc (BRL.N: Quote, Profile, Research) to sell a generic version in the U.S. market.

Bayer disagrees with the court's decision and said it will consider its legal options.

Bayer shares were down 4.5 percent at 47.96 euros at 1055 GMT, compared with a 0.3 percent decline in the pan-European DJ Stoxx drug index .

Yasmin had sales of 321 million euros ($487 million) in the United States, the world's largest healthcare market, last year. Including other Yasmin franchises such as the lower dose version YAZ, sales amounted to 1.0 billion euros in 2007.

They were Bayer's top-selling drugs, slightly ahead of its multiple-sclerosis drug Betaseron.

Bayer, which invented aspirin more than a century ago, inherited the drugs following its 17 billion euro acquisition of Schering in 2006 to boost its healthcare business.

Analysts now fear the loss of the key patent on Yasmin could threaten its intellectual properties protecting YAZ.