Previous problems:

The FDA knows how hard it can be to close that door. In the mid-1990s, overriding the objections of public-health experts from the Centers for Disease Control and Prevention (CDC), the drug agency approved the marketing of two drugs, Baytril and Saraflox, for use in poultry.
Both are fluoroquinolones, a class of drugs important for their ability to fight the bacterium that causes anthrax and a food-borne bacterium called campylobacter, which causes a serious diarrheal disease in people.

Before long, doctors began finding fluoroquinolone-resistant strains of campylobacter in patients hospitalized with severe diarrhea. When studies showed a link to poultry, the FDA sought a ban.