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 dog. 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 already present when the product is applied, all of the ticks may not be killed within the first 48 hours.

Fipronil is mainly metabolised to its sulfone derivative (RM1602), which also possesses insecticidal and acaricidal properties. The concentrations of fipronil on the hair decrease with time.