Actually, all drugs intended for animals used for food have to be tested for residue issues- IOW, after the animal takes the drug, how long does it take to get out of the animal's system? For animals bound for slaughter, withdrawl periods can range from 7 to 60 days, and it's against the law to knowingly sell an animal for slaughter that could possibly have drug residues. If the drug is targeted towards pets (dogs or cats) this testing is not necessary, but the warning must be put on the label.