U.S. Magistrate Judge Theodore Katz ordered the U.S. Food and Drug Administration to begin proceedings unless makers of the drugs can produce evidence that their use is safe.

If they can't, then the FDA must withdraw approval for non-therapeutic use of those drugs, the judge ruled.

The FDA had started such proceedings in 1977, prompted by its concerns the widespread use in livestock feed of certain antibiotics - particularly tetracyclines and penicillin, the most common. But the proceedings were never completed and the approval remained in place.

"In the intervening years, the scientific evidence of the risks to human health from the widespread use of antibiotics in livestock has grown, and there is no evidence that the FDA has changed its position that such uses are not shown to be safe," Katz wrote.

The lawsuit was filed by environmental and public-health groups including The Natural Resources Defense Council, Center for Science in the Public Interest and the Union of Concerned Scientists in the Manhattan federal court in May.

The plaintiffs argued that using common antibiotics in livestock feed has contributed to the rapid growth of antibiotic-resistant bacteria in both animals and humans.

Antibiotic-resistant infections cost Americans more than $20 billion each year, the plaintiffs said, citing a 2009 study from the Alliance for the Prudent Use of Antibiotics and Cook County Hospital.

In his ruling, Katz ordered the FDA to follow through on the process it started in 1977 but only formally abandoned in December last year. The FDA said the proceedings were outdated and that it intended to pursue other regulatory strategies for coping with potential food-safety problems.

"The FDA has not issued a single statement since the issuance of the 1977 (notices) that undermines the original findings that the drugs have not been shown to be safe," Katz wrote.

The FDA could not be immediately reached for comment outside regular business hours on Thursday.

The case is Natural Resources Defense Council et al. v. FDA, in the U.S. District Court for the Southern District of New York, no. 11-3562.

(Editing by Edwina Gibbs)

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