Some active ingredients, particularly the macrocyclic lactone (ML) class of chemicals, can be used for treating external parasites, e.g. lice, ticks, buffalo flies or mange, as well as internal parasites. However, case studies in Queensland have shown that on farms where cattle were treated at frequent intervals for cattle ticks with a ML pour-on product, internal parasites, notably small intestinal worms (Cooperia species) and barber’s pole worms (Haemonchus placei), developed resistance to this chemical class. To reduce selection for chemical resistance and prolong the lifespan of drenches to treat internal parasites, the use of multi-parasite products should be limited, and tick control should be based mostly on products specifically intended for external parasites. Changing chemical group regularly, and considering other parasites when treating will help to reduce the build-up of chemical resistance.