Scientists fear the widespread use of antibiotics on the farm may be a factor in the rise of "superbugs" – bacteria that grow resistant to drugs, infect humans and defy conventional medicines.

Amid these concerns, the U.S. Food and Drug Administration has come under pressure to curb antibiotic use in farm animals. In 2003, the agency announced plans to evaluate every new animal drug based on the drug's potential to create superbugs.

But the FDA hasn't reviewed the vast majority of animal drugs now on the market, because most were approved before 2003.

Reuters found that the agency has evaluated such risks for only about 10 percent of the approximately 270 drugs containing types of antibiotics the FDA considers medically important for treating humans and are also used in chickens, pigs and cattle.

Overall, the FDA has evaluated the superbug risks for only about 7 percent of the approximately 390 drugs containing antibiotics that the agency has certified for veterinary use in chicken, pigs and cattle.

Since the 1940s, the animal husbandry industry typically has included low levels of antibiotics in feed, in part to promote the growth of animals raised for meat. (See related story, reut.rs/1tW5GMa ).

The FDA initiative to limit the risks posed by animal antibiotics took on new life last December. The agency issued a voluntary guideline calling on animal-drug makers to limit the approved uses of their drugs.

Aware that poultry, pork and beef producers were attracted to many antibiotics because of their ability to promote faster growth, the agency urged the drug makers to discontinue growth promotion as an approved use on labels of medically important antibiotics.

In a related initiative, the FDA by April 2015 intends to implement a "veterinary feed directive." The new regulation will require veterinarians to oversee the use of antibiotics available over-the-counter that are mixed into feed.

Today, all major makers of animal drugs that contain antibiotics, including Zoetis Inc and Eli Lilly & Co's Elanco Animal Health unit, voluntarily have agreed to start removing growth promotion claims on product labels, according to the FDA. Labels must be modified by December 2016.

As a result, 31 drugs have been withdrawn from the market by their makers, and manufacturers have changed the labels on two other drugs, according to the FDA.

Even so, the labeling changes leave a loophole. No matter what labels say, meat producers can continue to use existing antibiotics at low levels, so long as producers assert the drugs are used for treatment, control and prevention of disease.