Effect of formulation and route of administration on tissue residues and withdrawal times

The formulation of a drug can have profound effects on tissue residues and, consequently, withdrawal times (WDTs) in food animal species. The WDT is the time after completion of treatment needed for tissue concentrations of the drug and any metabolites to decrease to less than the tolerance value or drug concentrations considered safe for human consumption. A drug can have a zero WDT if, after administration, the concentrations in the tissues are less than the tolerance value. A more thorough review of WDTs and tolerances and how they are determined has been published. 1