21 CFR 556.1 - General considerations; tolerances for residues of new animal drugs in food.
(a) Tolerances established in this part are based upon residues of drugs in edible products of food-producing animals treated with such drugs. Consideration of an appropriate tolerance for a drug shall result in a conclusion either that:
(1) Finite residues will be present in the edible products - in which case a finite tolerance is required; or
(2) It is not possible to determine whether finite residues will be incurred but there is reasonable expectation that they may be present - in which case a tolerance for negligible residue is required; or
(3) The drug induces cancer when ingested by man or animal or, after tests which are appropriate for the evaluation of the safety of such drug, has been shown to induce cancer in man or animal; however, such drug will not adversely affect the animals for which it is intended, and no residue of such drug will be found by prescribed methods of analysis in any edible portion of such animals after slaughter or in any food yielded by or derived from the living animal - in which case the accepted method of analysis shall be published or cited, if previously published and available elsewhere, in this part; or
(4) It may or may not be possible to determine whether finite residues will be incurred but there is no reasonable expectation that they may be present - in which case the establishment of a tolerance is not required; or
(5) The drug is such that it may be metabolized and/or assimilated in such form that any possible residue would be indistinguishable from normal tissue constituents - in which case the establishment of a tolerance is not required.
(c) Any tolerance required pursuant to this section will, in addition to the toxicological considerations, be conditioned on the availability of a practicable analytical method to determine the quantity of residue. Such method must be sensitive to and reliable at the established tolerance level or, in certain instances, may be sensitive at a higher level where such level is also deemed satisfactory and safe in light of the toxicity of the drug residue and of the unlikelihood of such residue's exceeding the tolerance.