Ranbaxy Labs has suffered a setback in its attempt to launch its generic version of Pfizer Inc’s anti-fungal drug Diflucan in the United States. A U.S. court has upheld Pfizer’s six-month marketing exclusivity.

The U.S. drug giant’s patent on Diflucan, which garners sales of as much as $1 billion a year, expired on January 2004. The company had contended that it was testing the drug for use in children, and accordingly its patent had been extended for a six month period.

While a lawsuit by the Canadian company Novopharm had been unsuccessful, Ranbaxy had approached the Food and Drug Administration claiming that its version of the drug did not infringe upon Pfizer’s exclusivity period for the drug. In May 2003 Ranbaxy received tentative approval from the FDA to sell its generic version in four different version once the patent expired. U.S rules say that the first company to file for approvals and successfully challenge a patent on a branded drug gets six months of marketing exclusivity for its generic drug. Subsequently, Pfizer filed a suit challenging this attempt to get FDA approval. And keeping with the norms of the rather confusing world of pharmaceutical legalities, Ranbaxy responded with a counter suit.