Following administration of Stronghold, adult fleas on the animal are killed and no viable eggs are produced and larvae (found only in the environment) are also killed. This stops flea reproduction, breaks the flea life cycle and may aid in the control of existing environmental flea infestations in areas to which the animal has access.

Stronghold may be safely administered to animals infected with adult heartworms, however it is recommended, in accordance with good veterinary practice, that all animals 6 months of age or more living in countries where a vector exists should be tested for existing adult heartworm infections before beginning medication with Stronghold. It is also recommended that dogs should be tested periodically for adult heartworm infections, as an integral part of a heartworm prevention strategy, even when Stronghold has been administered monthly. Stronghold is not effective against adult Dirofilaria immitis.

If a dose is missed for heartworm prevention and a monthly interval between dosing is exceeded then immediate administration of the product and resumption of monthly dosing will minimise the opportunity of a development of adult heartworms. When replacing another heartworm preventative product in a heartworm disease prevention programme, the first dose of Stronghold must be given within a month of the last dose of the former medication.

Stronghold was administered at 10 times the recommended dose to cats and dogs and no undesirable effects were observed.

Stronghold was also administered at 3 times the recommended dose to breeding male and female cats and dogs, including pregnant and lactating females nursing their litters and no undesirable effects were observed.

Stronghold was also administered at 5 times the recommended dose to ivermectin-sensitive collies, and no undesirable effects were observed.

Stronghold was also administered at 3 times the recommended dose to cats and dogs infected with adult heartworms and no undesirable effects were observed.

In extensive field testing, no interactions between Stronghold and routinely used veterinary medicinal products or medical or surgical procedures were observed.