[Food toxicology: free residues, bound residues: respective toxicity]

Two types of metabolites can be expected in food prepared from animals or plants treated with veterinary drugs or pesticides. The first types are represented by the drug itself and/or its primary metabolites as free or conjugated forms. Except for highly lipophilic compounds, these metabolites generally appear early and are noncumulative, since they are metabolized and eliminated rapidly. These substances may have toxic properties and may be metabolized to highly reactive electrophilic intermediates. We consider that they may be potentially toxic for consumers. The second type include covalently bound metabolites which appear later. Their presence can indicate a potential toxic effect in the target species, although these compounds have lost their chemical reactivity. They therefore probably represent low toxicity for consumers, especially considering their low bioavailability, which has been demonstrated for many veterinary drugs and pesticides.