[Establishment of posology: the case of antiparasitic agents]

As generally observed for all veterinary drugs, the dosage regimen of anti-parasitic drugs must take into account both the therapeutic efficacy and the hygienic value of animal food products. However, the target of these drugs is not the host animal but parasites characterized by their particular life cycle stages. Consequently, anti-parasitic agents are generally either slowly eliminated or used as slow-release formulations both of which contribute to an extended tissular presence. The determination of the optimal drug dosage regimen is based on the study of a dose-efficacy relationship, the determination of the route and the frequency of administrations. Among factors affecting the anti-parasitic drug dosage regimen, differences could be related to physiological variations, such as interindividual variability, age- and species-related changes in drug distribution, metabolism or elimination, nutritional status, gestation and lactation stages. Finally, drug interactions due to associated therapy (anti-inflammatory or antibiotic drugs) or the existence of chronic diseases (eg, parasitism) could modify the normal fate of drugs with consequences on both their therapeutic effectiveness and the persistence of drug residues.