Permethrin is a powerful insecticide that is the active ingredient in many flea control products for dogs such as flea shampoos and flea medications.
It disrupts neurological functioning in insects like fleas by disrupting the sodium channels in the brain, and kills fleas and ticks on contact.
One thing that you should be aware of is that permethrin is toxic to cats and a flea product with permethrin is okay to use on dogs but you should never use it on a cat.
The insecticide is not harmful to most mammals, with cats being a very big exception, but kills insects indiscriminately.
Permethrin is used as an insecticide in many agricultural crops such as corn and wheat and not only kills the destructive insects that are targeted, but also kills many beneficial insects such as bees, etc.