The antiparasitic activity of ivermectin depends on the presence of an active drug concentration at the site of parasites location for an adapted length of time. Ivermectin interactions with another concurrently administered drug can occur. Concomitant administration of some drugs can increase the bioavailability of simultaneously administered ivermectin. This can, in some cases, become a useful pharmacological strategy to improve its antiparasitic efficacy and to delay the development of resistance in livestock or, in other cases, lead to adverse drug reactions and toxicities. On the other hand, other interactions can result in lower levels of this drug, determining that moderate resistant residual populations of the parasites may persist to contaminate pastures. The characterisation of ivermectin interactions can be used to predict and optimise the value of the parasiticide effects. This article reviews the pharmacological interactions of ivermectin in several domestic animal species.