Drug interactions amongst companion animal parasiticides

Having a pet cat or dog in the home brings many psychological and physical health benefits. These pets, however, can carry many ectoparasites, worms and protozoa that require treatment and control to maintain pet health and reduce zoonotic risk. This need has driven the development of a wide range of drugs and treatment options for use as parasiticides in cats and dogs. These will be licensed with a high margin of safety when used individually, but with new parasites being introduced to the UK, more pets travelling abroad and widening distributions of already established parasites, there is an increasing need to use drugs in combination. This article considers the commonly used parasiticides in cats and dogs and which are safe to use in combination.

When considering whether to use more than one drug in combination, safety should be the primary consideration. The safety of a product when used in combination may be specified in a data sheet. This implies that studies have been carried out and data assessed, actively demonstrating that these compounds may be used safely in combination with the drug in question. At the opposite end of the spectrum, the combination of some products with specific drugs may be contraindicated, indicating that there are data to suggest their use in combination is less safe than if used alone. In most cases, however, there will be no data either way and in these cases, a judgement call must be made by the veterinary surgeon as to the benefits of using these drugs in combination when their safety has not been established.