The U.S. Food and Drug Administration issued an order Wednesday restricting farmers and livestock veterinarians from giving livestock one class of drugs that are commonly used to treat infectious diseases in humans.

The class of drugs called cephalosporin is commonly used to treat pneumonia, strep throat, and urinary and skin infections. But farmers and veterinarians also prescribe the injection drug for illnesses in animals that are not  listed on the drug's label.

Federal regulators fear the more it's used liberally in animals, the higher the likelihood the drug will lose its effectiveness in humans.

Starting in April, federal regulators will require restricting so-called "extra-label" use in cattle, swine, chickens and turkeys to only certain instances in an effort to stave off the threat of antibiotic resistance in humans.

The decision comes after the agency announced it was withdrawing its plan to limit the use of antibiotics like penicillin and tetracycline that are used in the feed of healthy livestock.

Some experts say the current order may be part of the agency's effort to deflect criticism after its decision in December to back out of its previous plan targeting prophylatctic antibiotics.

According to the FDA, cephalosporins constitute less than .25 percent of antibiotic sales for use in animals.

"The big issue of antibiotic use in animals is antibiotics like penicillin and tetracycline," said ABC News' cheif health and medical editor, Dr. Richard Besser. "It's disappointing over Christmas the agency abandoned pursuing bans on these antibiotics in feeds."

But according to Dr. William Flynn, the FDA's senior advisor for science policy, cephalosporin is still one of many classes that can lead to resistance.

"The withdrawal of those outdated actions was not meant to be an indication that we're moving away of addressing that issue," said Flynn. "We're looking at this whole issue broadly, and are pursuing from a number of different angles."

The FDA planned to ban the popular antibiotic in animal feed in 2008, but pulled back after receiving pressure from livestock veterinarians at the time. The current order would still allow veterinarians, in limited cases, to prescribe the antibiotics for some additional infections not listed on the label.

"We are gratified that, with the exemption of extralabel use, that the agency has allowed veterinarians to use their clinical knowledge to diagnose and treat sick animals," said Gapz Riddell, executive vice president of the American Association of Bovine Practitioners, based in Auburn, Ala. "Our members will continue to use antibiotics judiciously and when needed."

Farms will still be allowed to use an older form of the antibiotic, called cephapirin. According to the FDA, cephapirin has not contributed to antibiotic resistance.