The global burden of ectoparasitic infestations is exacerbated by the lack of licensed vaccines, meaning safe and effective ectoparasiticide drugs are vital to their prevention and treatment. However, adverse clinical consequences of treatments for ectoparasites affect thousands of cats and dogs in the UK each year. The unpredictable outcomes of these treatments in some animals can be associated with undesirable consequences and there is a lack of knowledge surrounding their use. This article discusses indications of the major classes of ectoparasiticides used in small animal practice and highlights the types of adverse drug reactions associated with the parasiticides used to treat ectoparasite infestations in dogs and cats. Overall, the incidence of adverse drug events reported in relation to ectoparasiticide use, compared to the total doses administered globally, is small. The potential consequences for animal and human health of not using ectoparasiticides is likely to be more serious than the current rate of adverse drug reactions being reported. The benefits of protection from ectoparasite infestations will therefore outweigh the potential consequences of adverse events associated with such treatment.

The burden of ectoparasitic infestations is aggravated by the lack of vaccines and the inability to prevent the most serious vector-borne infections. This makes the administration of effective and safe ectoparasiticide drugs vital for prevention and treatment. However, adverse clinical consequences of treatment with ectoparasiticides affect many companion animals in the UK each year. In this study, information about the frequency of adverse drug events associated with the use of some ectoparasiticides in dogs and cats was collected, from data obtained from The National Office of Animal Health compendium datasheets and data available from published studies. Different ectoparasiticides were associated with different prevalences and types of adverse dermatological, gastrointestinal and neurological events. The heterogeneity between studies and the lack of field safety data for some products precludes head-to-head comparison of adverse events associated with ectoparasiticide treatment between studies. Therefore, caution should be exercised about making any conclusions when comparing ectoparasiticides. However, the data indicated an association between ectoparasiticide treatment and adverse events and demonstrated that this association can vary across different drug administration routes and classes. The low number of observations of the adverse drug events, relative to the number of administered doses, confirms that the health benefits of ectoparasiticides far outweigh the potential risks associated with their use. Further research is required on how adverse drug events can be predicted and prevented.