Pfizer, the world's biggest drugmaker, has filed a lawsuit to protect its patent over blockbuster anti-impotence drug Viagra, and thwart potential competitors.
A joint-venture of rivals Eli Lilly and ICOS said the lawsuit alleges that the proposed marketing of its potential Viagra competitor, Cialis, would infringe its newly issued "method of use" US Patent No. 6,469,012.
A similar suit has also been filed by Pfizer against Germany's Bayer, which plans to launch its own anti-impotence product, Levitra.
The legal action comes amid a fierce battle to dominate a market that analysts estimate will rise to between $3 billion and $4 billion a year by 2006 from $1.6 billion today.
Pfizer has received a broad US patent that it claims prevents companies whose products are made using the same active ingredient from entering the market until 2011.
Lilly ICOS and Bayer, which plan to launch their new products in the United States next year, said they will contest the suit and are confident they will prevail.