Fipronil is an insecticide and acaricide belonging to the phenylpyrazole family. It acts by inhibiting the GABA complex, binding to the chloride channel and thereby blocking pre- and post-synaptic transfer of chloride ions across cell membranes. This results in uncontrolled activity of the central nervous system and death of insects or acarids. Fipronil exhibits an insecticidal and acaricidal activity against fleas (Ctenocephalides spp) and ticks (Rhipicephalus spp, Dermacentor spp, Ixodes spp. including Ixodes ricinus) in the cat.

Fleas will be killed within 24 h. Ticks will usually be killed within 48 h after contact with Fipronil, however if ticks of some species (Rhipicephalus sanguineus and Ixodes ricinus) are present when the product is applied, all the ticks may not be killed within the first 48 hours.

In vitro, fipronil is mainly metabolised with subcellular liver fractions to its sulfone derivative. However, this may be of limited relevance ‘in vivo’ as fipronil is poorly absorbed in the cat. The concentrations of fipronil on the hair decrease with time.