Generic drugmaker Watson Pharmaceuticals sued the U.S. Food and Drug Administration, challenging a ruling against Watson's generic version of a big-selling diabetes drug, Actos.

Watson said that the FDA improperly denied the company's shared exclusivity on the generic version despite Watson directly following directions received from the regulator.

As a result of the FDA's decision, approval could be delayed for up to six months, Watson said in a statement.

Watson planned to launch its generic version of Takeda Pharmaceutical's Actos on Aug. 17 after a 2010 settlement with the Japanese drugmaker.

The first company that files for approval of a generic version of an off-patent drug typically gets marketing exclusivity for 180 days.

An FDA spokeswoman said the agency would not comment on ongoing litigation.

Actos had total U.S. sales of about $2.7 billion for the twelve months ending May 31, Watson said, quoting IMS Health data.