U.S. regulators said Wednesday that 25 out of 26 drugmakers that sell antibiotics used in livestock feed for growth enhancement have agreed to follow new guidelines that would make it harder for farmers to use their products to create beefier cattle, heftier hogs and other outsize animals.

The companies, including giants Eli Lilly and Bayer, have agreed to start the process of removing any growth promotion claims from their products' labeling, according to the U.S. Food and Drug Administration. Farmers who use the products for livestock growth promotion would then be using them for off-label purposes — and therefore illegally.

The FDA had announced the guidelines in December as part of an ongoing bid to stem a surge in new mutations of antibiotic-resistant bacteria that can affect humans. The drug companies are under no legal mandate to remove growth claims from their antibiotics — but if they do, that would make it illegal for farmers, ranchers and other agricultural groups to use the drugs for promoting growth in livestock, according to regulators.

"The FDA and drugmakers appear to have passed the first big test of the agency's voluntary approach," said Laura Rogers, director of human health and industrial farming for the Pew Charitable Trusts. But, she added, "there's a lot more to do."

Critics argue that the guidelines give drugmakers too much discretion in policing their own use of antibiotics and provide no mechanism for enforcement.

"This plan is likely to lead to label changes, not a reduction in use," said Avinash Kar, health attorney with the Natural Resources Defense Council.

Critics also say the FDA has done too little to enforce its rules. A report in January found that the agency allowed antibiotics to be used in feed even after it was shown that such use was causing the drugs to become less effective in humans.

The FDA’s decision announced Wednesday came after years of public health experts’ warnings about the emergence of new strains of bacteria that cannot be controlled by a wide range of current antibiotics. Some believe these superbugs have developed because of people eating meat containing antibiotics.

In its announcement, the FDA said the companies have also agreed to require that such antibiotics, which are typically added to animals' food or water, be made available only through a veterinary prescription instead of being available for sale over the counter at feed stores and other retail outlets.

The agency said there are 26 drug companies and 283 affected products or applications that could fall under the voluntary guidance.

Of the 26, Pharmaq AS was the only company that declined to follow the voluntary guidelines. Pharmaq makes an antimicrobial powder used to treat certain conditions in salmon, trout and catfish. The Norwegian company's product already is for therapeutic uses only but is available over the counter, according to nutritionists.

Al Jazeera and wire services