The duration limit—the maximum length of time a drug should be given—covers medically important antibiotics, which are those used for livestock that are also important in human medicine.

When such antibiotics are used for long periods, the risk of bacteria developing resistance to those antibiotics increases. As bacteria develop resistance to more drugs, treating human illnesses becomes harder.

In a notice announcing the final guidance, officials said it is “intended to mitigate development of antimicrobial resistance for these antimicrobial drugs.”

But public health advocates said the new policy represents a step backward. They note that the agency did not include an upper limit for all drugs, nor did it require companies to consider how duration limits might contribute to increasing antibiotic resistance that could harm humans. The news comes just weeks after FDA data revealed a 2024 spike in sales of medically important drugs for use in cattle, pigs, and poultry.

“While FDA claims the guidance is to mitigate antibiotic resistance, it allows drug makers to set durations solely based on animal health needs determined by the drug makers,” Steve Roach, the Safe and Healthy Food Program director at Food Animal Concerns Trust, said in a statement. “FDA could have chosen to set a limit consistent with its mission to protect human health as it has in the past, but decided not to.”

As part of a larger plan to encourage the responsible use of medically important antibiotics in agriculture, the FDA has been working for years to push manufacturers to create duration limits for older drugs. But more than a quarter of the drugs still don’t have them.

In 2023, the agency published a draft of the guidance, drawing pushback from public health advocates and members of Congress. The agency received more than 4,500 comments on the proposal, the vast majority of which were from advocates and individuals who “requested that FDA limit all durations of use to no more than 21 days,” according to the FDA.