21 CFR §530.21

1. (a)FDA may prohibit the extralabel use of an approved new animal or human drug or class of drugs in food-producing animals if FDA determines that:
   1. (1)An acceptable analytical method needs to be established and such method has not been established or cannot be established; or
   2. (2)The extralabel use of the drug or class of drugs presents a risk to the public health.
2. (b)A prohibition may be a general ban on the extralabel use of the drug or class of drugs or may be limited to a specific species, indication, dosage form, route of administration, or combination of factors.