Fipronil is a broad-spectrum insecticide that belongs to the phenylpyrazole chemical family. Fipronil disrupts the insect central nervous system by blocking the ligand-gated ion channel of the GABAA receptor and glutamate-gated chloride (GluCl) channels. This causes hyperexcitation of contaminated insects' nerves and muscles. Fipronil's specificity towards insects is believed to be due to its greater binding affinity to the GABAA receptors of insects, than to those of mammals, and to its action on GluCl channels, which do not exist in mammals. As of 2017, there did not appear to be significant resistance among fleas to fipronil.