The US Food and Drug Administration allowed more than a dozen antibiotics used in animal agriculture to remain in livestock feed — despite knowing that the drugs posed a "high risk" of perpetuating human antibiotic resistance.

Those are the conclusions from a new report by the Natural Resources Defense Council (NRDC), which based its conclusions on FDA documents obtained through a Freedom of Information Act request. The NRDC, an environmental advocacy group, has long argued that the FDA has mismanaged the approval and distribution of antibiotics in animal agriculture, even suing the agency on multiple occasions for failing to address "the potential human health consequences of feeding large quantities of antibiotics to healthy animals," in the words of NRDC executive director Peter Lehner.

Some of the drugs threatened to promote antibiotic resistance

According to the latest NRDC report, an internal FDA review sought to determine the safety of 30 different feed additives — all of them used not to treat illness, but rather to promote growth or prevent ailments. The review, conducted between 2001 and 2010, concluded that 18 of those additives were indeed risky to public health. It also found that inadequate safety information existed for several more. More specifically, FDA scientists concluded that some of the drugs threatened to promote antibiotic resistance, a growing and deadly phenomenon that's now blamed for upwards of 23,000 deaths in the US each year.

"This discovery is disturbing but not surprising given FDA's poor track record on dealing with this issue," says Carmen Cordova, a microbiologist and lead author of the new analysis, in a statement. "It's just more overwhelming evidence that FDA — in the face of a mounting antibiotic resistance health crisis — is turning a blind eye to industry's misuse of these miracle drugs."

"This discovery is disturbing but not surprising."

According to some experts, however, the NRDC report fails to acknowledge important improvements the FDA has made in recent years. Some of the drugs cited in the agency review, for instance, have been withdrawn from the market by pharmaceutical companies since the study wrapped up. The agency has also tightened its approval standards where new antibiotics for animal agriculture are concerned. And the FDA last year released new guidelines in an effort to phase out the use of antibiotics to promote animal growth — though the move was criticized largely because those policies remain voluntary.