In the European Union and elsewhere there is a requirement to ascribe causality to adverse drug reactions which occur in treated animals. In the EU, the ABON system of causality assessment is used but the assignment of causality assigned is not always self evident, and it may be complicated for a variety of reasons. In this paper, the approaches to causality assessment, based on a number of key criteria which examine the administration of the drug in relation to the sequence of ensuing events and the presence of biological plausibility are examined, along with the utility of using algorithms to facilitate this process. Unexpected adverse drug reactions usually require expedited reporting, depending on national or regional regulatory requirements. Again, deciding on what might constitute an expected (or unexpected) adverse reaction, particularly when a product may be intended for use in a number of species, and when within any one species a number of breeds may be treated, is not necessarily a straightforward issue. However, an approach to facilitate the decision- making process, based on a similar approach used in the pharmacovigilance of human medicinal products is discussed.