A very small number of dairy farms have been found using animal antibiotics in ways not approved by law, according to a new study by the U.S. Food and Drug Administration’s Center for Veterinary Medicine. Because antibiotic residues immediately appear in milk, dairy cows are only supposed to receive drugs when they’re known to be sick. Regardless, FDA found that 0.78 percent of samples tested positive for drugs not legally allowed in milk due to safety concerns. The agency called the results “encouraging” and said that they illustrate the high level of safety in the U.S. milk supply. The study, which began in 2010, took nearly 2,000 samples of milk directly from cows at farms categorized into one of two groups: Dairies that had been warned about abuse of antibiotics in the past and a control group of dairies that had never received warnings. (FDA routinely sends out warning letters to dairies if they find excessive levels of drugs in a dairy cow sold to slaughter.) Researchers tested for 31 drugs that have no established tolerance level in milk. Any sample with a positive test result would be considered in violation if found during routine milk testing, but all testing was performed anonymously for the purposes of the study. The results: 11 out of 953 samples (1.15 percent) tested positive from dairies that had been warned about abusing antibiotics in the past, while 4 out of 959 samples (0.42 percent) tested positive from dairies that had never received warnings. One of the samples came back positive for two separate drugs. Every shipment of milk leaving a farm is tested for only six major drugs that are approved in limited amounts. The 15 positive milk samples contained drugs that are not on that list for routine testing because they are not approved to be used on lactating dairy cows. The majority of the drugs found are allowed to be used on cows when they’re sick, but they’re not allowed in the milk supply. When dairy cows do require antibiotic treatments, they’re typically culled away from the herd and must undergo a drug withdrawal period to make sure that they don’t contaminate the milk supply with any drug residues. But even in the rare cases where drug residues make it into milk, the levels are almost guaranteed to be too miniscule to negatively impact a human, said veterinarian Michael Payne in an article for dairyherd.com. “Tolerance levels in meat and milk are established by FDA and based on animal studies designed to show no harm to humans exposed daily to those concentrations for an entire lifetime,” Payne wrote. “Importantly, even those calculated safe levels are further divided by an additional safety factor of either 10 or 100.” Despite the evidence of an overall safe milk supply, FDA said it will consider modifying testing samples of milk supplies from farms that have been found with illegal drug residues in their dairy cows. The agency also plans to work with regulators to update the milk testing program to include a greater diversity of drugs.