Most drugs are accumulated in excretory organs such as the kidneys and liver , and they are found in higher levels for a longer time in these organs than in other tissue. Due to a continuous excretion of the drugs from these organs, tissue levels are usually rapidly reduced after the drug is no longer fed or injected. The rate of excretion of a drug and its end products from animal tissues must be established before it can be considered for approval by the Food and Drug Administration. Then withdrawal periods that are sufficiently long enough to permit complete or near complete elimination of the drug from animal tissues , milk or eggs are established. Once a feed additive is approved it means that the FDA considers it completely safe for use in animal feeds without unsafe levels of residues occurring in the animal tis sue or its products if the withdrawal period is observed and it is fed at recommended levels.
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Department of Animal Science and the Agricultural Experiment Station, South Dakota State University
Fox, D. G. and Bailey, J. H., "Complying With Feed Additive and Drug Withdrawl Periods" (1971). South Dakota Cattle Feeders Field Day Proceedings and Research Reports, 1971. 22.