How safe is your meat and milk? That depends on who you ask. The use of antibiotics in farm animals has long stoked controversy, especially among those who claim that they have unintended consequences. Now, reports Helen Branswell for STAT, rules for antibiotics in animals just got stricter in a bid to protect humans.

A new rule about antibiotics in food animals is now in effect, writes Branswell. The first part of the rule is a ban on the use of drugs deemed “medically important” for humans in food animals for growth promotion. For those that require such treatment, a veterinarian must be present. The second part of the rule suggests that producers label antibiotics to make it clear they should not be used to promote growth in food animals.

Antibiotics have only been mass-produced since the 1940s, and soon after scientists realized that adding antibiotics to animal feed made them grow faster. As Maureen Ogle writes for Scientific American, a surging demand for animal products spurred research for products that could improve animal nutrition. Once antibiotics began to be used in animal feed, animals survived at a higher rate in crowded conditions and could be brought to market more quickly. The supply of meat grew and prices dropped.

But all was not well on the factory farm: As antibiotics became more and more common in livestock feed for both the treatment of diseases and to spur on growth, antibiotic resistance began to spread. Livestock began to use more antibiotics than was available to humans, and illnesses that once responded readily to antibiotics like tetracycline and penicillin became harder to cure.

The reason lies within animals’ guts. When animals frequently are treated with antibiotics, the bacteria inside their intestines dies. Antibiotic resistant bacteria is left behind—and it can run riot without other bacteria to fight it. As the CDC notes, the resistant bacteria can then make their way into the food supply during slaughter, through water contamination and through the meat and milk of animals.

As antibiotic resistance becomes a worldwide concern, attention has turned to the animals who help fuel it. The FDA already limits the use of antibiotics in dairy cows, determining thresholds for drugs and testing their presence in milk. Despite ever-stricter rules, the agency has found that some illegal antibiotics are making their way into the food supply.

Now, writes Branswell, the FDA is looking at other ways to limit the use of antibiotics in animals. Labeling medically important antibiotics in feed could deter farmers from opting for the feed, and putting antibiotics under the control of vets rather than farmers will likely drive the cost of using the drugs up so that farmers are discouraged from using them. But a loophole in the new regulations still allows antibiotics to be used for an unlimited amount of time—a practice that, opponents say, is “growth promotion by another name.”

Despite evidence that using medically important antibiotics in farm animals harms humans, their use is still rampant. According to the Union of Concerned Scientists, around 24.6 million pounds of antimicrobials are used for animals who are not sick per year—eight times more than are used to treat humans. But industry secrecy and a lack of rigorous data means that it’s hard to quantify just how many antibiotics are used in farm animals. Until the industry becomes more transparent about how it uses antibiotics, it will be difficult for the public to judge the new regulations’ effects. In the meantime, humans have reason to curb antibiotic use on farms: the long-term health of people and animals alike.