Wyeth (WYE) has filed a lawsuit against the U.S. Food and Drug Administration seeking to reverse the agency's recent approval of a generic version of the company's antibiotic Zosyn.

Wyeth claims the generic drug can't be safely used in the same manner as the branded product, due to interactivity with another drug that is sometimes used in conjunction with Zosyn, and would seriously endanger patients' health. The suit was filed in U.S. District Court for the District of Columbia.

The FDA approved on Sept. 16 a generic version of Zosyn from Orchid Chemicals & Pharmaceuticals Ltd. (524372.BY) of India. Orchid is to have a 180-day period of generic market exclusivity, and has joined with Apotex Inc. of Canada to sell the drug.

Wyeth, Madison, N.J., had previously asked FDA not to approve generic versions of Zosyn because they were based on an earlier formulation of the drug, and Wyeth changed the formulation in 2005. But the FDA recently concluded that the formulation wasn't discontinued for safety and efficacy reasons, clearing the way for generic approvals.

Zosyn had $1.3 billion in sales last year. Wyeth has agreed to be acquired by Pfizer Inc. (PFE); the deal is expected to close by the end of the year.

An FDA spokeswoman said the agency doesn't comment on lawsuits.

-By Peter Loftus, Dow Jones Newswires; 215-656-8289; peter.loftus@dowjones.com