Watson Pharmaceuticals is being sued by Pfizer over its plans to market a generic version of the US giant's erectile dysfunction drug Viagra.

Watson has filed an Abbreviated New Drug Application with the US Food and Drug Administration seeking approval for its version of Viagra (sildenafil). Pfizer immediately filed a lawsuit in a New York court looking to prevent Watson from commercialising its product prior to the expiration of a key patent.  

Pfizer's lawsuit was filed under the provisions of the Hatch-Waxman Act, resulting in a stay of final FDA approval of Watson's ANDA until November 6, 2013, the generics major says, "or until final resolution of the matter before the court, whichever occurs sooner, subject to any other exclusivities".

Despite being on the market for over a decade, Viagra is still a big earner for Pfizer. For the year ending April 30, it had total US sales of just over $1 billion, according to IMS Health data.