Antibiotics Medication that kills or slows the growth of harmful bacteria. Farmers give antibiotics to treat cows that are ill, just as humans sometimes need medication when they are sick. Milk from a cow being treated is separated to ensure it does not go into the milk supply. All milk is tested for the most commonly used antibiotics upon delivery at the dairy plant. A 2015 report from the U.S. Food and Drug Administration determined the nation’s milk safety system is effective in helping to prevent drug residues of concern in milk, and no milk contains antibiotics at the retail level.