[Covalently bound residues: the case of anthelmintics].

: In target species, most anthelmintics produce unextractable residues, which include covalently bound adducts, and a radioactive fraction, possibly incorporated into intermediary metabolism. Both are responsible for the very long half-life of total residues in tissues. Due to the analytical difficulties encountered in distinguishing between these two fractions and to the controversy about the toxicological significance of bound residues, discrepancies exist in the determination of withdrawal periods for a given drug. Withdrawal time varies from one country to another, depending upon the regulations of the different national agencies.